It’s definitely free labour. Even if you don’t mind doing it, it’s still that. As for the rest, I’m sympathetic to your point of view. I have used reviews and occasionally left reviews on products I liked particularly. However, online reviews are so gamed at this point that I don’t think anyone ought to feel pressured to leave a review like they try to do with the emails OP is receiving. It’s one thing to like a product so much you are moved to leave a review. It’s another to be asked for it every time you purchase something.

Good user reviews simulate word of mouth advertising which is the most valuable sort. They want free labour from you to help with that. This would be a great application for a spam filter.