As someone else mentioned, you weren't stuck there. You did have options you didn't know about to leave.

As for the 30 minute penalty in general, it's there to put some weight behind the decision to leave. People still dip on something just because they don't like it, but we would definitely see an increase if people could just leave the duty without any penalty at all. And there is an aspect there that it's what you signed up for. You joined the random matchmaking system the game provides where you can't really choose what you're going to get. The game doesn't trap you there. It does give you the option to leave. It just doesn't let you make that decision frivolously.