Timmy told me that the guy doing the designing/branding was going off an older MacBook for consistency and didn't think to check with the later models. This was missed when going to production so they just had to go with it.
IMO, I think it's so people know what it is. The MBPs are pretty obvious to anyone that knows anything about Apple laptops. If it's metal and thicker than a pencil, it's a MBP.