Apple doesn't want to ship a new version because they don't want people to see how they've modified bash. I don't know what patents or top secret features they've added to make their version of bash incompatible with GPL, as they're allowed to ship GPL code in a closed source product just fine as long as they ship it as a separate binary, but it's more important than their users' convenience apparently.
Also note that Microsoft ships entire Linux distros without having a problem with GPL licensing.