Mike has talked about it at length. Basically, the decision was partly because he liked what RR had signed when he was younger. Besides, as he's said, believe it or not, the RR guys enjoy good music. I believe Mike has gotten some rare vinyl through them. And lastly, Mike was so thrilled to sign with them, he didn't really read the contract, and admitted it might have been something about a gay porno starring Opeth for all he knew.
I got WS SE the day it came out. The SL SE was unavailable anywhere in my vicinity on its release date. Didn't get it for weeks. I'm pretty happy with RR, actually. And mostly, so long as they stay out of the way (which it seems like they do), I really don't give a flying fuck.