Surely its an unwritten rule, like waiting for someone who has a mechanical during an attack?
No. It's a
written rule, where if you ask or accept an illegal feed, you get a penalty. Both Porte and Froome took a penalty, I wouldn't be surprised if there were financial as well as time penalties.
And no, this isn't a horrendously disrespectful breach that is like stabbing Le Tour in the heart with a stale baguette. It's a minor infraction, it was pretty obvious, and they got busted for it. BFD.