I'm no expert, but I have to assume that part of Apple's deal with the labels is that the RIAA won't petition Apple for information from Match. Which leaves the RIAA to use the same tactics they've been using to identify illegal music sharing.
I agree, Match doesn't make the initial acquisition of illegal files legit. Not much different than stealing a car and getting it painted.
It would make no business sense for Apple to give information to the RIAA about your iTunes library. If Apple ever did that it would piss off so many people it would probably bankrupt Apple as a company since they would lose all trust.