It does seem a bit unfair, but at the same time the reviews aren't just a measure of how much the bride and groom will enjoy the place, it's also a measure of how much everyone else does as well.

Perhaps "many of your guests will hate this place" is a valuable signal to couples picking venues.

Honestly, if you've been a legitimate guest of a particular venue IMO it's perfectly fine to leave a review about it. The people that foot the bill aren't the only ones who get to chime in.