I posted a quick Twitter comment about this today and want to explain it a little more. There are a few Youtube beauty gurus I know of who routinely accept gifts from their subscribers. Some of them don't solicit these gifts, so I want to be fair in pointing that out. However, they do accept and say they use the products, sometimes reviewing them and sometimes not.
The reason I don't like the concept is because a Youtube guru with a high subscriber and view count has easy access to many items for free. This is a nice benefit to being popular (anyone see Xsparkage's open box video?), which any savvy product reviewer will understand and work to his or her advantage. The companies should be donating any items to the reviewers; the subscribers shouldn't be donating as much (and especially not young teen subscribers). Like the business that promoting products is, this is an equitable exchange.
A Youtube or beauty reviewer with a large following can easily reach out to companies, show his or her page or channel, and ask if the company would be willing to donate items in exchange for those products being featured. The companies and the reviewers benefit equally; a subscriber sending gifts out, while it's generous and thoughtful, isn't benefiting other than knowing she sent a gift. This may be enough satisfaction for some subscribers, which is great.
In a few too many situations, I see gurus welcoming the gift-giving by sharing their P.O. box and saying how "sweet" people are for giving them things. It's absolutely a nice gesture from the subscribers, but some gurus aren't being nice in exchange. They're taking advantage.
A Youtube beauty personality I respect highly was receiving unsolicited gifts from fans. She posted a comment immediately saying how appreciative she was, but she asked that people donate to charity in lieu of giving her things going forward. She noted in her life she's been blessed with a lot, so she'd like things to go to others who are in much greater need.
Keep in mind: Highly popular reviewers are often sponsored; they are already being paid very well to do the work they do. Some of them benefit in profit-sharing because their channels are so highly viewed. They have other features enabled that bring in profit to them.
None of this is wrong: If you are spending a considerable amount of time reviewing products and sharing your knowledge, earning a living off it is a reasonable, fair, and legitimate goal. The amount of work that bloggers Temptalia (Christine) and Nouveau Cheap do is absolutely full-time work and has my respect; they regularly post 3 or 4 new blogs per day. I just dislike seeing some other reviewers taking advantage of their fans' goodwill.