On the waste issue, that was exactly what the ASI article the Rogdcam linked was about. The article makes the point that the USPS has "...continually challenged the industry to provide more environmentally responsible products." (Of course, they are required to by Executive Order. Which is good.)
Plus, it cites "cost savings" of PSA stamps for a variety of reasons.
As for the sanitary advantages that PSA may offer, I would agree that not having to lick a stamp certainly eliminates the possibility that a pathogen may be introduced into the body by that route. And let's face it, most people did lick stamps unless you were maybe sending 100 invites; then a sponge may start looking good. It makes me wonder about the actual vs perceived risk of a WAG stamp causing illness. I doubt the risk is very high. But then, why risk it? I guess as a mailer, I like PSA. As a collector, I don't. I admit that sounds stupid.
Companies switching to a more costly system: Well, most probably don't do that purposely. But sometimes, things can happen that makes you wonder "why'd they do that?"