I think it's like drugs for a lot of these youtube reviewers, airguns or otherwise. They start off innocent enough, and just having some fun, but then money becomes involved if their channels gets popular and vendors start offering them more and more freebies and incentives. Eventually the vendors just start paying them outright for "reviews", and the reviewer is then essentially just working for the vendor(s).
A common question that reviewers (again, airguns or otherwise) publicly get asked in the comments is along the lines of "why are all of your reviews positive???". It's a good question, and the most common answer is "because I never bother making/publishing a video for the gear that is crap, that's why you never see it". Not really sure if I buy that...