I think requiring explicit permission like this is what journal and book publishers normally do, because for them there’s a serious chance of being sued for using any content that someone else has copyright to without permission (probably technically including anything with a “© Google” in the corner of the map). So maybe this particular professor is trying to follow best legal/ethical practices by doing the same thing.
In reality I don’t think people usually worry about using Google Maps screenshots in much of anything that’s not going to be for sale (or otherwise a major publicly-available product). But in this case I assume it’s going to depend on how much of a stickler your professor wants to be about the technicalities as they understand them.