I agree there is nothing stopping someone from starting an instance with ads. It’s just that the overlap of people that agree with such a thing and are enthusiasts of free and open source software must be very niche.
I’m also not suggesting the devs could or should do anything about it. I’m just saying it isn’t something they would do. It would be worth determining which license is being used, but I doubt it is one that prohibits commercialization of Lemmy.
At any rate I don’t think such an instance would survive as the type of user that would agree with both ads and Foss is limited. It is unlikely to ever become a real issue.