It's absolutely accurate to portray them that way; sure, there are some used, no-papers bikes that are not stolen, but people willing to buy such bikes are what creates financial support for bike thieves, since all stolen bikes sold meet that description, and if people weren't willing to buy bikes in that condition stolen bikes couldn't be sold.
Imagine if people had to drive around in creaking beater cars with leaking roofs because you couldn't store anything nicer anywhere but a garage with surveillance, and even then if someone finds out you have a nice car in a garage, there will be a breakin. That's pretty much the situation with bikes, and police simply shrug at what in most cities is very much organized crime and a huge quality of life issue.
All stolen bikes also have two wheels. I guess buying a bike with two wheels also creates a market for stolen bikes?
This is ridiculous. No one without papers can sell a bike according to you. I get that bike theft is a problem but you don't solve it by blaming innocent people. A lot of legitimate bikes are sold without papers.
The bigger point is that it's unreasonable to paint all people who buy used bikes without papers as supporting bike thieves. Anyone buying my bike would not get paperwork with the sale, but the bike is not stolen.