Because the guild consists of many authors, unless it’s spectacularly unsuccessful. You’re right about “Starbuck Coffee,” however - that would make more sense.
Why?
Ah, so a place that more than one person walks is a “pedestrians walkway,” not a “pedestrian walkway.”
A train that takes more than one person from the countryside to their places of work is a “commuters train,” not a “commuter train.”
A person that uses more than one computer in a day in order to create software is a “computers programmer,” not a “computer programmer.”
In other words, what I’m saying is that the plural is not mandatory. For some uses, it’s merely habitual.
So, then, why am I a member of the “Newspaper Guild” when there is more than one newspaper involved?
Ah, so a place that more than one person walks is a “pedestrians walkway,” not a “pedestrian walkway.”
A train that takes more than one person from the countryside to their places of work is a “commuters train,” not a “commuter train.”
A person that uses more than one computer in a day in order to create software is a “computers programmer,” not a “computer programmer.”
In other words, what I’m saying is that the plural is not mandatory. For some uses, it’s merely habitual.
Harrods is taken from the surname of the founder though it does have an ‘s’ added (the founder was Charles Henry Harrod) so should be listed as Harrod’s. It’s all very confusing.
Some googling reveals that it wasn’t always this way, in the case of Harrods it lost it’s apostrophe ‘Some time between 1895 and 1902’ and shortly before that Lloyd’s Bank became Lloyds Bank. Also, ‘In 1891 the US Board of Geographic names called for an end to possessive forms in place names’ causing Henry’s Fork to become Henrys Fork and a host of other changes.
So it seems sometime around the turn of the twentieth century people lost all interest in the possesive apostrophe and dropped it from pretty much all business names.
It may be possessive, but if you gonna drop the object it’s possessing, why not drop the apostrophe too. Besides, the Germans don’t use it. This may be what’s affecting English usage.
Looks like the apostophes from Starbucks, Harrods and whatever else wound up here. I’m guessing you really meant to use quote marks on what appear to be quotations or cites that are missing their links.
I often use ’ ’ instead of " " when quoting things, I don’t know why 
As for the first apostrophe in “it’s” should that not be there? I thought that there would be a possesive apostrophe there because the apostrophe belongs to Harrods?
The possessive of “it” is “its.” No apostrophe. Just like “yours,” “his,” “hers,” “ours,” and “theirs.”
Two British examples, FWIW: Sainbury’s emphatically retains the apostrophe (despite the parent company being ‘J Sainsbury plc’). On the other hand, ‘M & J Boot, Herbalists’ had morphed into ‘Boots the Chemist’ by the early 1900s, including the still-used logo. I can’t find any relevant information for Barclays or Debenhams…
While the above explanations for apostrophe-dropping are no doubt true, there is a secondary explanation for a number of businesses:
There were a number of 19th Century businesses, on both sides of the Atlantic, which were founded by father and son, or brothers, and used the plural of their surname as a part of their business name: Franklin and Jacob Smith would found “Smiths’ Dry Goods” using the plural possessive. Many such businesses were in time contracted to the simple plural, easily mistaken for a singular possessive.
My hometown had a chain of three corner stores founded by a family of Lebanese immigrants who became pillars of the community, which only recently went out of business with the retirement of the last family member to run one, which my parents knew as “Habeeb Brothers.” By my generation, though, they’d become “Habeebs.” No apostrophe; you are invited to guess case and number for that business name.
What is the North American POV on placenames then? Typing in the first possessive US name I can think of, and doing it without an apostrophe, Google scolds me: “Did you mean: Martha’s Vineyard?” The apostrophe version gives about four times as many hits.
In my part of the world, the Geographical Names Board of New South Wales insists that placenames do NOT have apostrophes. Hence, Tom Uglys Point, Johnsons Creek, etc.
The U.S. Geological Survey has the same rule. I believe it was instituted because on their maps they wanted to use the apostrophe as a symbol for a physical feature and they didn’t want any confusion with place names. However, an exception was made for Martha’s Vineyard.
Similarly, the rule was set down that “-burg” is the standardized spelling. An exception was made for Pittsburgh.
I remember reading in an old catalog that Lands’ End was supposed to be “Land’s End.” A printing error in the first catalog results in the company name being spelled “Lands’ End.” They decided it was easier to change the spelling of the company name than reprint the catalog.
I can’t find confirmation on the web.
It’s none of those. It might be Penneys or Penney’s.
You’re mistaken. The use of the apostrophe called for in this case (to which Exapno was alluding) is Authors’ Guild. This is the proper way to express the possessive of a plural. (I don’t know what makes you think that the guild doesn’t belong to the authors.)
As to the OP, I’d assume that marketing and graphics people prefer to have names and logos that are as clean as possible, and feel that punctuation marks add distractions and the possibility of errors or onconsistencies on the part of employees and the public. I’ve done this myself in marketing pieces I’ve created, sacrificing perfect punctuation for simplicity in a headline, for instance.
As Exapno said, “It’s simpler, cleaner, and doesn’t give rise to error.”
Caught me - yes, if you’re going to use an apostrophe in “Authors’ Guild,” it belongs after the “s.”
The same thing that makes me think that the “dairy products aisle” in the supermarket doesn’t “belong” to the dairy products. It simply contains dairy products. They don’t own it.
Just to clarify: if a guild is created by a group of authors, and they own the organization, or if the members of the guild, by virtue of joining, obtain some ownership rights, then “Authors’ Guild” is correct.
If, on the other hand, a wily entrepreneur who’s never penned a line of prose or poetry in his life decides to form a guild for authors, and collects fees from them in return for, say, being their advocate, then “Authors Guild” is correct.
Barclays Bank was started by the Barclay family, who were quakers, in Norwich. It is after one of this family - or maybe all of them - that Carrow Road’s Barclay Stand was named.
Barclays’ first bank building is still in operation in Norwich.