The DM was wrong. It was nice of you not to make a fuss at the table, but there's a difference between simply pointing out an incorrect ruling and bringing the game to crashing halt to argue about it for more than half a minute. While the argument you suggest is valid in a way, "because magic" could be used to justify almost anything. You'd be better served having the relevant text at hand and citing it.
Worst case scenario, the DM likes to make spontaneous house rules to shut down his players. More likely, he just didn't understand the rules and you missed an opportunity to improve his system mastery. Don't hesitate to speak up when the DM makes an incorrect call, just try not to be a douchebag about it.