Because, as it turns out, while science has shown time and again that it does not cause arthritis, it CAN sprain a joint - like any other ligament sprain. And it really sucks if you are, perhaps, an aggressive knuckle-cracker and find yourself in just this predicament.
Prescription? Ibuprofen. And rest; lots of it. For example - wrap it up so you remember not to use it, thus letting it rest. Sheesh.