Yes, the royalty rates that Spotify pay out are abysmal: pennies for thousands of plays last I heard. I've no idea what kind of agreement that O'Donnell/Salvatori have with their distributor, but they are relatively experienced, relatively successful and relatively independent, so I would assume the terms are pretty favourable. My guess is they are aware it's there and are OK with it.

Perhaps they think it's a lesser of two evils (get a few extra pennies from Spotify rather than have someone pirate it) and think it'll be promotion for albums that are past their primary sale period.

: but I realized recently I'm not sure Marty & co. still even get
: money for this music,

Yes, they would do. Music copyrights are extremely long:
Composition copyright: until death of last remaining author + 70 years. (neither have died yet)
Recording copyright: 50 years (Still another 37 years to go for Halo 1)

: and if I did buy the soundtracks I would probably
: get them cheap on Amazon, so they would almost definitely be used,

Pfft, even buying new is pretty cheap TBH.