The thing is, a reviewer receiving somehitng doesn't invalidate the review. My editor has a strict policy..if we don't like a product we contact their rep and let them know our concerns. If the company doesn't respond well or fix issues that might be possible then the review doesn't go up and the item is sent back. When you get tickets to somehitng and you don't like it, we do say the negative aspects. Most people know if they are going to a certain event they are already fans or have interest in it.
Professional bloggers do this as a full time job, it takes a tremendous amount of effort and time to constantly uodate your sites on all platforms, manage content and writers, deal with sponsors and product reps, etc. They deserve to make some money or more likely compensation in the way of products or experiences, because they are indeed providing the company with nearly free advertising. This doesn't mean a good blogger will only review thigns they get for free,bit is usually a mix.
There is nothing nefarious about a blog that states they received a product. This is how ALL reviews are done of media and most consumer review magazines, websites etc. Some are big enough to buy everything but most are not. Believe me there's a lot of junk that gets sent back or even reviewed poorly. All reviews of books, music etc, are given copies, film critics given screeners,etc. It's has always been this way this isn't new.