Why would you want to download a Triple A exclusive anyway?
That seems like an arbitrary line to draw. Unless you're really into having a game on your shelf or getting collector's edition stuff, why should the pedigree of the game matter?
If I had to guess why it's two parts, it's probably a weird PS3 and/or PSN file size limitation. I seem to recall some Unix file systems (maybe ext3?) having a 16 GB file size limit if they're configured a certain way.