Are you of the opinion that because a behavior is legal that necessarily makes it right in all cases? If so, I suppose that is where I disagree.

You seem to be interpreting a license as a legal formality that shouldn't be taken so literally.

It is a form of very explicit and literal communication. Unfortunately you sent an unambiguous message to the world inviting them to do this.

(Disclaimer that I briefly worked at Google years ago)

EDIT: Well OP did not give them permission to alter the license. Sorry this happened to you. https://news.ycombinator.com/item?id=35791737

If their behavior annoys you, why not DMCA takedown the repo in retaliation for them stripping the copyright header? It'd be within your rights since they violated the license and sends a message they can't ignore.

Otherwise if you're upset they otherwise used code you released under a permissive license that's a personal problem. You signaled to the world that you'd relax your copyright under a permissive license and don't like the results? This is like gifting something and being mad about how the gift was used.

As others have pointed out you don't seem to grasp why licenses exist in the first place? No org or individual in general wants to play guessing games and risk liability. It's entirely on you to signal how you want your work to be used.