Like getting caught stopped AT&T?? Didn't they make what the NSA asked for legal after-the-fact AT&T got caught?
There is a technical issue that Apple doesn't support redirecting messages...although they could allow the DEA to have an additional iMessage device. Apple probably "could" do it.
The REAL issue is that there is NO LEGAL MANDATE for Apple to do so. Aple running a chat program is legally no different than YOU running a chat program. Apple is not a telecommunications provider or an ISP, nor do they get the legal protections of those classes. If you are going to hold Apple to this standard, then you would have to tap EVERY instance of Blackberry Messenger or Microsoft Lync... Even PRIVATE ones operated by your company.
The DEA is hijacking the discussion to get something that has Congress has not wanted to pass multiple times. The DOJ has been able to push telcos and Microsoft around because of monopoly rules and applying laws based on their "class" as common carriers or ISPs. This is the classic "there should be a rule" because they have got what they wanted from "lawbreakers" so now non-lawbreaking companies should comply too.