He described Spotify's rates as "pitiful" in the Tweet here:
https://www.northstandchat.com/threads/martyn-ware-heaven-17-v-rockstar-grand-theft-auto.409552/#post-11285637
Yeah, fair enough. He did mention royalties - but I took this as an explanation is that $7500 was a flat, one-off fee, and can't rise. (Even if it was used in a trailer?)
I think your final line here sums up exactly why he's insulted. Imagine someone talking your most famous song - a huge hit...
He's making a stand and encouraging artists to put value on their work - and not just accept the promise of 'expsoure' and, maybe, millions of streams... which are worth little anyway.
Reasonable imo.
Edit: In fact...
Tweets tell the story.
Twitter/X is now full of 1000s of American no-marks saying things like...
Fair play to Martyn for sticking up for the value of his work.
Apparently GTA VI will have 400+ songs. Probably would lead to exposure among da kidz. Then again, so has this spat.