Yes I didn't mean that the disclosure was the problem. It's the incentivised reviews that are the problem because you really can't give an honest opinion even if you want to. You didn't pay for the item like a normal reviewer and you have an implicit incentive to give a good review so you aren't reviewing from the same point of view as the reviews people actually want.

It's the same reason many companies have policies that employees can't accept gifts. In theory they could have a rule "you can accept gives but don't let it affect your decisions", but people aren't robots. You can't turn that off.