It is more complicated, because otherwise it can be abused by just telling your friends in the party to withdraw on your behalf, thus multiplying the amount of strikes a given person effectively gets ("punishment splitting"). Being in a party would thus be a way of bypassing the strike rule.
Pro-tip: Most people who were repeatedly queueing and withdrawing before were trying to fish for in-progress runs, not just trying to troll other people. Which is something you already can't do in a party anyway.

I really don't think NOT penalizing the whole party would result in a bunch of trolls returning to the DF queue with an army of other people just so that they can keep withdraw spamming.