> All kinds of small businesses are routinely run out of residential zoned properties.
Yup. And they typically have something in common - they don't bother the neighbors or create an atmosphere that is noticeably different than that of a residential neighborhood without the home business.
Work from a home office? No one will notice (other than the fact that you're home a lot). Teach piano lessons? As long as your students' parents don't park in front of other houses, probably not a big deal. (But people will notice.)
Want to run a metal shop out of your garage M-F? You'll get shut down because it's a disruption. Want to do metal shop projects on the weekend? No big deal, as long as you don't start too early and bother the neighbors.
> "Renting a room" seems like the kind of small business that's ideal for this situation - it takes no more infrastructure to house a vacation rental than it does to house a longer-term resident.
It's not about the "infrastructure" (when reduced to simply "a place to stay"), but about the externalities of everything that comes with frequent turnover of for-pay visitors. A vacation rental inherently means frequent turnover of residents, whereas "renting a room" means having a long(er) term resident with much less turnover.
> Leaseholders can also throw parties and be disruptive; there is an existing legal process to deal with noise complaints.
Yup, and it's easy to do because a leaseholder, well, lives there so the law can take its course. A vacation renter is gone pretty much before you can pursue any existing legal processes apart from calling the cops to come by and tell them to turn the music down. So, the AirBnB renter doesn't have to live with much/any consequences; the AirBnB "host" doesn't necessarily have consequences since they'll say "I won't rent to them again"; but the folks in the neighborhood do suffer from it.