I think this opens up a great discussion in regards to contractual agreements, "hired hand" drummer, or "band member", royalty agreements, etc.
Although we do not really know the agreement they shared re: songwriting/publishing on various songs, if it's not written down, it's not valid. Perhaps he was promised points on various songs.
I think that drummers need to account for what they bring to a song re: groove, "direction", feel, etc. Is the groove or drum part to a song as important as a melody and lyric? In todays pop market, I think it's 50/50.
In general, if you are being paid to do a session, you are being paid to create a part for a song, much like a guitar player doing a part or solo. If the artist already has a groove and direction they want you to play, that's one thing. If they say "what do you think the groove/feel/direction should be on this song?", then you are being asked to contribute to the writing of that song, in my opinion, and a "point" or royalty of that song should be negotiated. If the artist starts the song as a shuffle, and you change it to an up tempo funk groove, and they like it, and it's a hit, then you've created a very important part of what makes that song work.
I will be following this closely, as I think we all should, as it may open up new avenues for us "behind the scenes" musicians!