I disagree that it is important to know that they were paid if they genuinely like the product. Any hint of it being advertising immediately discredits the content, and there is no need for that to happen in the case of a genuine endorsement.
Sure, in the perfect world, we'd fire up the intent detector and know. In the real world, everyone would claim 'Oh, but I do love the product.' and the end result is it's unregulated and people get tricked.
It's still not the same, though: I genuinely like a ton of products, but most of them I wouldn't actively promote if I weren't being paid to do so. There's still a difference between someone off-the-cuff mentioning/plugging a particular product they like, vs. someone being paid to do so, even for a product the use and like regardless of the payment.
But why do you think that any hint of being advertising discredits the content? Surely if there's reason to think that, you'd want to know if content is discreditable? Or do you just think that the masses will mistakenly discredit content if they know money was exchanged?
If it's a genuine endorsement then money wouldn't be changing hands.
That isn't remotely close to being true in all cases.