Imo that's pretty unethical. If you don't want ads, use an app where the maintainer has decided not to include ads or not to charge.
But don't take something from a sole-developer (meaning in this case labor is actually getting all the fruits of their labor instead of a CEO), use one of the other options instead where the developers aren't using it as their main source of income.
That's not the actual reason. Hexbear was openly advocating for their "army" to brigade other instances once it was federating. It just so happens that the basis of that brigading was going to be political.
Lemmy.world pre-emptivly decided it wasn't worth the hassle of having to deal with that.