I haven't really been following the sidechain developements, but my understanding was that redemption from a side chain would be two phase.  The person unpegging the funds provides a proof that they have locked the funds on the side chain and are eligible to withdraw the funds, plus they put up a bounty.  Then there is a time-locked period where others can collect the bounty by providing a fraud proof, that the locked funds given in the proof have actually been double spent.  This two phase system doesn't violate this rule.