As a kid, I remember going to the dump, and there were always a few bikes discarded... it isn't a new thing. But, at that time, they were mixed in with the refuse. Generally Murrays and similar bikes. But, often in good condition.

One thing that gets me. The local thrift stores (St. Vincents, Goodwill, etc) all have big garbage bins.

If they can't sell it... it goes into the trash.

Unfortunately, there is a grey area where stuff isn't worth $20, but it would get snagged for free (which might compete with their other business).

Most bikes they sell have 2 wheels, etc... But, for many of the bike enthusiasts... well, at least for me... I look at a bike as more of a collection of usable parts. So, if it has a good frame or good parts, I could care less if it was complete... that is if the price is right.

In fact, some of the bikes you're seeing at the dump may be there because they've been refused by the thrift stores.