U.S. is properly written with periods because it is initials for United States.
like the U.S. State Department uses it.
Lazy people often omit the periods in U.S. My English teacher loved docking points for this slip up. 
But UK doesn’t seem to use periods at all? United Kingdom NOT= U.K.?
At least, nothing pops up in google.
Whats the straight dope?
There is no requirement for anything of this sort. You can use either mode, style guides might say you have to use one. IME most Americans leave them out in casual typing/writing.
UK English leaves out the periods where US leaves it in, e.g. Doctor becoming Dr and Dr. Respectively. This is because the UK style only puts a period if the word is truncated and the last letter is removed, e.g. street --> st. in both countries.
That said, this may be a coincidence and have nothing to do with the cause of your observations, and there is a different reason.
Amazing how things taught as rules in my 1970’s English classes are now considered style guides. We had cast in stone rules. Kids these days have it so easy. At least we had bell bottom jeans. 
All my teachers counted points off for not using periods in abbreviations or initials. Trademarks were an exception. Dr Pepper never uses a period.
I do recall the Brits don’t use periods after abbreviations.
Nor do they use apostrophes in possessive business names.
I’m pretty sure women get periods regardless of what country they’re from.
Really? Because in one of Terry Pratchett’s books, it’s a minor plot point that a business does not use an apostrophe, and this is seen as a bit unusual.
Oh, I’m sure various contentious stylistic prescriptions are still taught as conflicting cast-in-stone rules to schoolchildren, no worries there. But we adults outside that racket can know better and see things for what they really are.
I would think because U.S. might be mistaken for the word us. But UK is not a word at all
Then you recall wrongly.
Not really true at all, but dependent on the specific business. See for example, Heal’s, Sainsbury’s and Waterstone’s.
British English does tend to use fewer full stops (periods) in abbreviations than American English. The style of most newspapers, for instance, is not to use them in initials. So we use ie, not i.e., eg, not e.g., US and UK, not U.S. and U.K., etc (not etc. 
). Similarly, it’s Mr AN Other, not Mr. A. N. Other.
I think it’s part of a trend towards less fussy writing. Another example: acronyms (pronouncable abbreviations) tend to be written with just an initial capital, rather than all caps. So whereas in the 1950s we might have written N.A.S.A., and later switched to NASA, now we write Nasa.
I’ve noticed that in British newspapers it’s now the norm to read that people have “Aids” rather than “AIDS” (or A.I.D.S)
Actually, since it’s not a proper noun, I see no reason to preserve the initial capital – “aids.” Just like “radar” and “laser” and “scuba.”
The straight dope is that this has nothing to do with “laziness.” It’s a matter of style.
This answer could be made a sticky. If you learned a rule in high school English, it’s wrong.
Of course if I were a high school English teacher I would tell you miserable brats it’s my way or the F-way. You don’t get to argue style with me.
In the adult world, there are rules only for formal grammar - many of which are furiously argued about in specific situations. Everything else is subject to opinion. That’s why there are so many competing style guides.
I’ve mentioned before that when I sold my first book I was told to use the Chicago Manual of Style. (Nonfiction books are sold before they are written, unlike most novels.) My head in the clouds, I eagerly rushed out to buy the book. Then I got a letter from the publisher, giving me the 50 ways their house style differed from Chicago style.
My head returned to earth. I shelved the Chicago Manual and said to myself, just write and let the copyeditor figure it out. Because no matter how much work I put into it, I would have to unlearn everything for the next editor. It wasn’t worth it.
It’s a British thing. The Master speaks: Is it true the British have abolished punctuation? - The Straight Dope
How do you know that it’s the last “T” that was removed from “street?” How do you know that it’s no different from the “doctor” case you mention?
When I first worked as an editor in the book publishing business, I was surprised at how much effort went into fixing minor style issues. Consistency is what ultimately matters, in my opinion, rather than one style over another. If you want to use serial commas, two spaces after every period, and capitalize “Internet” in your book (or website), just make sure you do it throughout.
Agreed. Consistency is far more important than any individual point.
Except for two spaces after a period, which should be a firing squad offense.
Anyone want to take a crack at why they don’t put a comma after ‘i.e.’ or ‘e.g.’? Not saying it’s universal, but one of the subtler differences in style.
because it would then be e.g., and i.e., which looks ugly.