Looking for why this is upsetting. I don’t know any reason for it.
Are you sure it’s truly upsetting to anyone?
My guess it is just a way to identify and thrown scorn on new arrivals and outsiders, who use a different term for the city than long time residents would use.
Some people take exception to the shortening of their name as a nickname. They find it denigrating and demeaning. People from Frisco are some of these people. 
Let’s say you have a friend who goes by the name of William. You start calling him Bobby. He doesn’t like it, but you persist.
Same thing.
That’s because the proper form is “Billy”.
Because Frisco is in Texas, not California.
If the history of language is any guide, some idiots used the term and the term became identified with idiots.
I didn’t know it upset folks.
It’s more of a shibboleth.
On the East Coast, don’t say “New York City”, “Washington DC”, or “Philadelphia”…rather, say “New York”, “DC”, and “Philly” respectively.
Herb Cain, the beloved San Francisco columnist, wrote a book called “Don’t Call It Frisco.” He said:
Moved to IMHO.
Colibri
General Questions Moderator
This. No San Franciscan is going to tolerate being mistaken for a city in Texas !
Nope. Frisco Colorado
It’s like people calling Chicago Chi-Town. Only tourists and asshats say that. It’s not upsetting in an offended way, it’s just annoying and identifies the sayer as an idiot.
Got it in one. I’ve lived here (SF, which is ok to say or write) for nearly 40 years, and the only thing that ever happens when someone says Frisco (which hardly ever happens anyway) is a snicker and a small cringe, followed by gentle correction if you care enough about the person to do so. No, Herb Caen, it doesn’t “remind” anyone of anything, it’s just the sort of thing one associates with noobs and visitors.
Now those folks born in San Francisco, which I wasn’t, may have a different perspective. I find them generally a little more persnickety about such things, and also more apt to bore you with talk of how everything has changed so much. To which my attitude is, time passes, things change, get over it.
The Herb Cain part seems reasonable but a few of the reactions I’m seeing in this thread seem a bit over the top. Philly isn’t considered a problem, most people in New Jersey don’t even mind Joisey which seems to be a strictly negative term. We take it as the joke it is. I don’t think Frisco actually has any negatives connected to it. Its just short and easy.
Does anyone call the Windy City Chi-Town? I never see or hear that one.
Let’s say I know three guys named William. One likes to be called Bob. Another Will. The last wants to be called William. Are the existence of Bob and Will to be taken as evidence that William is being a jerk by wanting to be called by the name he likes?
That’s right. Everyone knows it’s “The Chi.”
You can tell someone’s not from San Francisco if they call it anything at all. Locals just call it “the city.” But I wouldn’t say it really upsets anyone.
They pay a lot of money to live there. They pay for four syllables, dammit, so don’t shorten it to two like it’s Fresno or something.
The City is NYC of course. 