I think it's the economy that is contributing to this. This is happening a lot with horses too. A lot of people are unloading their horses to auctions where they are shipped to slaughter in either Canada or Mexico. It's awful! The backyard breeders who produce truckloads of poor quality animals are flooding the market with horses and causing such a large glut of horses that no animal will sell now.
Same with cats in my area. The shelters are packed and part of the problem is people not spaying or neutering. I saw an ad from someone wanting to get rid of their male cat...why? because they didn't want their female cat to get pregnant anymore. All they had to do was get their animals fixed and this would have been prevented.
I think one aspect is that the animals themselves are so cheap to buy, especially cats which are very often free, that people with poor incomes get them, and then can not afford to properly care for them, including spaying, neutering, vaccinating, worming etc.
My personal opinion is that if you can not afford the typical expenses (fixing, vaccinations, quality food etc.) of owning an animal, don't get it.
Urban Coyote