I guess I don't see how having to visit openstreetmap.org or any other site instead of .de if you want to look at a map for a few days is a "kick in the shins", especially since "providing a map viewing website" is not the goal of the OpenStreetMap orgs, neither the Foundation nor the local chapters. (one certainly can argue if maybe they should do have that goal, but they explicitly don't, regularly repeated when people suggest adding end-user features to the site)

They represent OSM in Germany, they think it's in OSM's best interest to draw attention to this issue, they do so on their own website, leaving all tile servers etc alone. I can see being momentarily annoyed at it, I can see disagreeing with them (politically or if that's a useful protest form), but I don't see anything that would count as a gross violation of trust or something that's out of scope for a local chapter to do.

Hm, I'll think about it. Have an upvote.

You contributed the data so that they - and everyone else - can do what they want with it as long as they stay within the terms of the license. I don't see that they are violating the license by choosing to render that data a certain way (in this case as a black tile).

German OpenStreetMap is not a third party who is only bound by the copyright license though - it's a contributors' community with various explicit and implicit expectations. See e.g. https://wiki.openstreetmap.org/wiki/Code_of_conduct

Parent never claimed that the license was violated though. You can be unhappy about something legal.