Whether or not I know how to run a hotel is irrevelant. The driving point I'm making is that customers do have a right to complain. Simly put:
1) Clients have a right to complain about their experience, no matter their booking arrangements. Such a complaint on a review site is perfectly valid. A hotel cannot reply to the review saying "they booked through an agency, so *shrug*", and it's settled. Other potential customers would not appreciate that attitude. Whether or not that's how hotels operate that way is not important -- a complaint in the lines of "I received poor service" is valid and provides true value to future potential customers.
2) Clients are not paid to do the work for management in reporting issues. Whether or not you may get a benefit from doing work for management is besides the point: the customer isn't signing up to do such work. The client expects good service independent of whether or not they do work for the owners. Management from various industries that value providing good service pay outside companies to evaluate their offering -- they don't expect the customer to do free work.