Who knows, though I always thought that it was rather odd that OpenTTD was on Steam. I'm not sure whether that's because it is an open source remake or because you had to own the original for the graphics/sound assets back in the day. (Apparently that changed over 15 years ago!)

Even if Atari's lawyers were involved, it may have been a friendly exchange. The post claims that OpenTTD was available on Steam for 5 years. That is more than enough time for them to apply legal pressure. It's also worth noting that the open source version is still available from the project website, as are the open assets.