> edit: what if they were being paid $20 a day for being tables, or coatracks? Would it be exploitative yet?
How about this:
In Poland we have people called colloquially "strzałkowi" ('arrow people'), who are working by literally sitting for a whole day holding a big arrow-shaped sign that directs to a bookstore/Subway (restaurant)/whatever. They are just sitting there, whole day, with an arrow, earning laughtable money, most likely as a way for companies to place signs where they usually would not be allowed to. Oh, and they are not even homeless, they're usually students who can't find any better job, because the job market for unqualified labor is kind of saturated. And suddenly, nobody complains about demeaning jobs and exploiting people.
I'd rather walk around with a wireless transmitter any day. Those wi-fi homeless people are at least providing some value to society.