The edge case is that A follows B, then A deactivates his account, then B switches her account to protected, then A deactivates account (within the 30 day window since deactivation). Now A is a bonafide follower of B (because he followed her before she switched to protected) and can thus view B's protected tweets.

I don't disagree that Twitter might as well patch up this corner case, but I think it's an extremely narrow corner case that shouldn't inspire much worry or outrage.