One reason I can think of is with regard to confidentiality. A lot of AI services are controlled by companies in the US or China, and they may not want military research to leak to these countries.
Classified project obviously have stricter rules, such as airgaps, but sometimes, the limits are a bit fuzzy, like a non-classified project that supports a classified project. And I may be wrong but academics don't seem to be the type who are good at keeping secrets nor see the security implication of their actions. Which is a good thing in my book, science is about sharing, not keeping secrets! So no AI for military projects could be a step in that direction.
Classified project obviously have stricter rules, such as airgaps, but sometimes, the limits are a bit fuzzy, like a non-classified project that supports a classified project. And I may be wrong but academics don't seem to be the type who are good at keeping secrets nor see the security implication of their actions. Which is a good thing in my book, science is about sharing, not keeping secrets! So no AI for military projects could be a step in that direction.