Your definition of "easy" is only actually easy if we ignore the little problems of detection, enforcement and proof. Anyone can call the city on illegal residential property uses today, but obviously, that approach isn't working I can flip through the listings of any newer residential building in SOMA, and see 50% of the units occupied by startups.
As for tolerance: what is acceptable in small doses becomes pathological when taken to extremes. Just like AirBnBs, it's not an issue when a handful of apartments are being used for commercial activity in a healthy market. When you have a huge percentage of renters doing it in a housing crunch, it's a concern.
As for tolerance: what is acceptable in small doses becomes pathological when taken to extremes. Just like AirBnBs, it's not an issue when a handful of apartments are being used for commercial activity in a healthy market. When you have a huge percentage of renters doing it in a housing crunch, it's a concern.