If someone is in a position where they are being offered money, it means that the party making the offer sees them as a viable marketing outlet. Marketing is a typical and often sizable expense for most companies. They can send free product and hope for a review or mention, but they may have reason to assume that many other companies are doing the same, and have no reason to expect that every one that does so will be chosen for a review/mention - if only because of the sheer volume of submissions. They'll be out product, and have no guarantee of a marketing opportunity. However, if they offer payment, acceptance of payment ensures that their product will be chosen. There is no requirement for a favorable review, just that one does occur. The fact that they are willing to risk a negative review shows that they stand by their product and are confident in its quality. In either case, the adage states that any publicity is good publicity. It isn't an indication of shady dealing by the company, but of the high perception of the reviewer as someone who is so well respected that they must be overwhelmed with submissions, and unable to cater to them all. They are paying for a service that they value, instead of hoping to receive the service for free via lottery.
The question pertains to "a reviewer" - someone established as a respected source for reviewing products. If it was an unsolicited offer to someone who has no history of reviewing for a testimonial, then it would be suspicious, but even then it would depend on context. If it was a "pepsi challenge" type scenario, say a live stream from an event where they offer cash to random people to complete a taste test, then offer 2 unlabeled bottles, and asked which was better, and it turns out to be "new company" juice vs "established company" juice, and the reactions were unedited - where is the harm? Is the money needed in this case? Probably not. Vapers like trying new juice. But simply offering the money is part of the marketing.
I find it odd that without context, the prevailing view is that any monetary exchange corrupts the system.
