Reviewers will get lots of that stuff free if they're popular enough and willing to give a good review no matter how bad it might be. Everybody can be one, if you got time on your hands and enough cash to get products by yourself till you're popular enough later on, and find lots of followers then you're at a good starting point.
I personally have purchased plenty of full lines myself but not as purpose of reviews. Some people can afford it, but either way there's nothing wrong with buying, as well as getting them for free in my opinion, as long as it doesn't affect their own opinion, and that's the issue with lots of reviewers nowadays. I do trust some of our VU guys here tho, when it comes to reviews, don't matter to me how they got it, if it was free I'm glad because they do deserve it.