Diana only stepped out after it was clear that Charles wasn't going to stop. Anyway, she's dead now and beyond any consequences this world could possibly give her, while Charles and Camilla aren't. They don't get a pass on publicly celebrating their prolonged infidelity just because the third party didn't react in an ideal manner when she was alive. Charles and Camilla are indeed fortunate, far more fortunate than either of them deserves.
If Charles' character is so weak that in his thirties he couldn't tell his parents, "If I can't marry the woman I want to, then I'll stay single," then it isn't strong enough for him to wear the crown. OK, the late Queen forgave him and kept him on as her heir despite the weakness he's shown absolutely no sign of maturing past. That's fine, and he's certainly not the only morally dubious man-baby to ever ascend the throne. But it should have been at least as low-key an event as his second wedding and not a lavish public celebration, and she shouldn't have been crowned at all. I realize that my opinion doesn't matter to them in the slightest, but there it is.