“Parallel construction is a law enforcement process of building a parallel - or separate - evidentiary basis for a criminal investigation in order to conceal how the investigation actually began.”
Why? They've been using parallel construction to avoid violating NDAs and giving up their capabilities for years. http://www.wired.com/2014/03/harris-stingray-nda/
1) Police don't care that APPLE doesn't want to reveal its methods, and 2) Unlike the Stingray, it wouldn't be illegal for them to use iMessage evidence in court if Apple provided it to them (especially with the use of a warrant!)