I have no... NO ... Problems posting reviews on products I've purchased. However, when I review Anker products from PU/PD I sometimes get stopped, and told..... You can only review stuff you've bought (in as many words!)
So I wouldn't say I have issues with Amazon reviews, but i don't say I got it free.... Maybe that's why I don't get a problem? IDK
Unless it's a rule to say it's a freebie, I don't, as my review is the same regardless of purchase price. For Argos, when I review, I do say it's a free item, but that's their requirement and the review is just for the Argos website.
(Argos is a UK catalogue/online shop, that does its own testing program)