I still do NOT think there is any other protection on these discs. This could be an issue in the Mac operating system like Albert stated, but I don't believe Netflix DVD's have extra copy protection.
Some discs do have more structural protection, but it won't be something only seen on the Netflix editions.
If you were using Windows, I'd recommend getting AnyDVD, DVDFab Passkey, or the free version DVDFab Passkey Lite and running one of those programs in the background to break encryption. This would eliminate the possibility of encryption being the problem. But none of these work on a Mac unless you want to run a VM or dual boot with Bootcamp.
What we cannot recommend is using a decrypter that works in OSX to rip the DVD's to the hard drive and play them from there. Netflix stopped selling old copies of DVD's back in 2008, so general consumers can only access their rental discs. And the rules of the forum prohibit us from discussing backups made from rented material.