Fretting/pressing buttons isn't actually as demanding as strumming, which really needs limb independence. The strumming hand is the "rhythm" hand and it's a lot easier to keep time with your dominant hand.
Of course this is the exact opposite of what I was saying with the Rock Band drums, but then they are two completely different instruments, and fake simplified versions of real instruments, to boot.
Also, I play real guitar left-handed as well, and have never played real drums. If you've been playing real guitar right-handed you're probably already used to playing rhythm with your right hand. (Though you probably would have learned faster with your left!)