[fulafel]

People use dead mammals for things like leather, bones, fat, feeding other animals/humans, science experiments, ornaments/status symbols, tallow, etc which you can't mine. The AI might be in a hurry (eg be in competition or war with humans other AIs) and trade off long term cooperation prospects for getting an upper hand in a fast paced emergent situation.

I'm not sure getting sorted into two piles, beasts of burden and recycled biomass, is really a win scenario for humans either.