DRM was never a consideration for ALAC since Apple never sold tracks in ALAC. My understanding was that Apple developed ALAC instread of using FLAC because they were concerned about possible patent problems with FLAC.
I heard the same theory about patents but after they reversed engineered it, the creator of FLAC pointed out it was effectively a superset of his codec, so any patent affecting FLAC would impact ALAC too, they'd only increased their exposure to patents.
Other theories included better PPC support, and better designed to be decoded in low power hardware like the Airport Express.
To be honest some combination of NIH syndrome and residual squeamishness about using open source code seems far more likely.
