[PantsOnFire]

This rule is in Robert's Rules of Poker and there is a good reason for it.

Player C is in effect calling Player A's raise. Player B also in effect called Player A's raise plus put in the rest of his chips which are not enough be considered a full raise. So when it gets back to Player A, he hasn't been raised and he can only complete the partial bet of Player B to continue in the hand, just like Player C did.

In your case the amounts are quite large. But imagine if Player B had exactly 5000, the amount of Player A's raise. Then Player C can call and the betting is over. But what if Player B had 5005 and Player C calls? Does it seem fair that when it comes back to Player A he can reraise again just because Player B made it 5 more to go because he's all in?

So the rule is clear, if Player B goes all in and that doesn't add up to a legal raise amount, then Player A can only call.