In speaking with Paul Brown of Recycle Ann Arbor, I found out a little more about the reasoning behind the sudden change in policies of these chains. As I should have guessed, it's economics, pure and simple. Logistics are also a problem. Buyers of recycled plastic purchase it by the ton. As you can imagine, it takes a lot of plastic bags to make up a ton.
Contamination is another problem.
People leave things such as paper receipts, wrappers, etc. in the bags when dropping them off for recycling, causing a problem for the recycler. According to her, there is a definite market for these bags. A company called Trex manufactures decking material out of recycled plastic, including bags. The decking is actually stronger and more resilient than wood. Mr. Mike Vanutu of Trex confirmed Ms. Rackley's assertion that Trex does need more of these bags for their manufacturing process.