Musk is an absolute sociopath, but there's actually a logic to this.

Apparently the US has extremely tight export controls for telecomm tech used for war, and Starlink was concerned that by Ukraine using it in an offensive way it would result in the US or other countries classifying Starlink as military tech, thereby limiting where they can export it. That would be really bad for Starlink, of course, which is why they specified at the beginning that Ukraine should only use it for civilian goals (hospitals, schools, government, etc).

I assume the contract between Starlink and the Pentagon covers that, but I haven't researched that far.