It seems unlikely to me that the OSMF did something shady in exchange for the promise of 6,000 Euros from Cesium.

The OSM Buildings folks called themselves "OSM Buildings", using the OSM trademark. They offered a commercial service. The OSMF was OK with this. It seems the OSMF is OK with a lot of things -- including permitting Cesium to use the OSM mark in their own commercial project.

Live and let live, may a thousand flowers bloom, etc.

Rather than reflect on the fact that their name was not really theirs, but relied on the goodwill of the OSM mark, the OSM Buildings folks decided to ragequit and shut down their project, dramatically claiming that their name was "seized by a multimillion dollar company"?

The whole thing is bizarre to me.