Can you Complete this Raise?

[DeepCroak]

Interesting situation I hadn't seen before. I was just an observer in this hand.

3/6 Hold'em, 4 players to the turn.

Player A bets $6.
Player B raises $3 to $9 all-in.
Player C calls Player B
Player D tries to complete player Bs raise to $12

At this point, the dealer informed player D that because player C had only called, and not completed Player B's raise, player D could only call $9, and could not raise. If player C *had* completed the raise (and I understood what the dealer was saying correctly), Player D would have been able to call $12 or raise to $18 at his option. But because player C only called $9, player D only had the option to fold or call $9.

This surprised me - I had expected that Player D (or player A, for that matter) would have had the option to complete the raise. But I was just an observer, Player D backed down and called without involving the floorperson, player A called $3 more, and the hand moved on to the river (where a side pot *was* created).

The dealer seemed quite confident in his ruling, so the possibility that he was in error seems low (though not zero). Assuming the dealer was correct, is this a common rule, or is it unique to where I was playing (Bay 101)? Any difference between B&M and internet? Just curious.

Thanks,

DC