Why is it ok that I can sneak attack a gibbering mouther, or a bone devil, but not an ooze or a skeleton?
Asked and answered already- in every edition previous to 4Ed, sneak attack has been couched in language of targeting vital spots, which is not quite synonymous with "weak" or vulnerable spots. Vitals, at least to me, implies that the creature has organs that are essential to its continued operation.
With negative energy as its sole animating force, a skeleton (really, any undead) has no vitals, unless you're arguing that a rogue should be able to disrupt the skeleton's..."nexus of unlife."
An ooze generally has no true organs either- they generate what organelles they need when they need them and in some sense, the nucleus is an undifferentiated mass of genetic info. The rogue and everyone else are essentiallly aiming at the same targets- protoplasmic pseudopods reaching out to strike or the center mass.