The Law should be applied equally to all.
My sympathies may lay with Ms. Letourneau, but as I tried to say earlier, the lines must be drawn.
As far as "the difference" goes, you can't rape the willing. I doubt it required much if any manipulation on Mary K's part to arouse Billy's interest. It’s also doubtful that any physical or long-term psychological damage was done. I doubt Billy will feel any shame for his participation, caused by societal reactions to what happened to him.
Letourneau broke parole and met with Billy. For this, you’re right; she is where she should be. For his sake she needed to be kept away from him until he could make a decision about their “relationship” as an adult.(IMO)