That's a good question, I honestly don't know. My guess would be that MFI is essentially a checklist of all supported magnetic functionality, and therefore the experience Apple can promise in its marketing (ie, in addition to collecting fees, they don't want a user to blame Apple when a non-approved case doesn't work). I'd wager that a non-MFI case with magnets in the right places would still work; I don't think constraining to pre-approved RFID codes or anything like that.