A lot of the "homeless" guys in my neighborhood are actually "semi-homeless". If they get a steady job for a few weeks, they rent a room or an apartment. When work gets slow, they get behind on their rent, and end up back on the street. Often, a reliable bike is essential for getting to a job.
Luckily for them, Goodwill and other resale shops in the area sell bikes cheaply. Pawnshops in my area charge about $75 to $125 for all of their bikes, whether it is a 1995 Cannondale, or a 1989 K-Mart bike. At the re-sale stores, those same bikes might sell for $10 to $50...lesss than a tank of gas for some vehicles.
I donated a 1980 Schwinn road bike to a resale shop in my neighborhood. It was a nice bike, but it had an annoying "clicking" sound near the bottom bracket. Three shops and a bunch of money never "cured" that "clicking" noise. Later, I went by the resale shop, and they had priced the nice Schwinn at $20...I was tempted to buy it back.
And, during "Spring Cleaning", Houston has "heavy trash days" where folks can leave out sofa's, TV's, and, (gulp) bicycles for the City to haul away. I often see in the piles of junk, a "last summer's" K-Mart or Wal-Mart bike. It might have a flat tire, or broken chain, or balky shifter. They were "shocked" by the $30 repair estimate at the LBS ("A $30 repair...you're joking....I'll just go back to Wal-Mart, and for $75, get a NEW bike").
So, there are lots of ways to get a bike cheap other than to steal one. A college student who bought a 2004 Cannondale for $200 from the guy he met at a club is as likely to be riding a stolen bike as that unemployed construction worker down on the corner. Lots of ways to get a bike...