The fact is that the point of making some special, magical creature immune to +4 weapon (but even +3) is to make such creatures impossible to be damaged *not* by "normal" ones (attacks) but more than that by any not powerful magical attack (+4 attacking cratures/weapons should be extremely rare...)
A bear, a common animal, no matter how many hit dice, should not be able to damage them because of their nature.
Even if that +4 at HD 10 is not meant to enhance Thac0 or inflicted damage, it still seems to overpower non magical creatures. For consistency then, as Galactycon said, humanoids of average-high levels should be allowed to hit with fists at the same way and that doesn't sound so good...Persoanl opinion of course!