Transients living in your midst! It sounds more like you pay extra to be away from the poors.
I'd be shocked if there were any doorman buildings that didn't explicitly forbid subletting (or AirBnB-style leasing specifically) in rental/association agreements.
In either case, the main point is that lease agreements (and in many cases, even the law, as we saw in Santa Monica) don't stop people from doing what they want to do. I don't know why you think the parent comment has anything to do with poor people; sometimes a person just wants to have a good idea of who's living in their building, regardless of wealth.
That's an overly simplistic view of the housing market, but yes, that's a major factor in real estate pricing.