Because there is something to gain from positive reviews, that is monetary or career wise. Just because you admit bias, doesn't absolve you from the appearance of, what I think, constitutes as unethical or at least ethically grey, using your platform to promote your friends. And remember, I said they are pretty ethical, I'm just saying if you want to be above reproach there is a way in doing it. I mean, I would think the simple fact that there are people who have problems with it, should infer that there is at least some perception of impropriety from reviewing products that could quite possibly benefit your friend, financially or career wise. While the statistical reality of this is low, if your goal is to be ethical, I do think it would be a way to achieve this.
They have no obligation to follow ethics(or at least the philosophy behind what being ethical is) that are not under law, so they can do what they want. I'm just saying that would be the best approach, imo. It's the same reason a lot of companies dont allow family to work in the same office if one of them is in a higher position, there is an implication of potential favoritism vs another group. If they want to embrace that they favor their friends in the industry, that is fine, but I don't think that means they are above scrutiny, either.