Adding your own garbage to someone else's URLs is in fact the problem. Could they handle your garbage better? Sure. Is your garbage still a problem? Yes.
Postel's law worked OK when people operated in good faith. But today the internet is full of abusers. Rejecting requests that aren't exactly what they should be is probably the best policy now.
Postel's law is typically stated as "be conservative in what you do, be liberal in what you accept from others". It's unfortunately common for people to ignore the first half and hallucinate a third clause demanding that the recipient stay silent about the errors they receive.