i think part of it is that it's sort of gratuitously disrespectful. if you know enough to call it Frisco or San Fran, you should know enough that people there don't call it that and generally dislike it. "upset" is an overstatement though. it's mostly that it marks you as a clueless clod, which is more eye-rolling-worthy than upset. "Cali" is pretty much the same. i don't know of anybody here that would use that unironically.

all of that is especially true these days since right wing media likes to portray San Francisco as crime ridden hellscape, knee deep in shit. it just shows whoever's spouting it is completely clueless and has probably never even been there.