I think the problem isn't that they're doing both of those things at the same time. The problem, instead, is that the third thing they're doing at the same time is issuing guidelines, (hopefully?) backed by their ranking algorithm, written specifically to combat this type of behavior.

If it were a free-for-all, sure, no problem. But Google doesn't want other parties doing this. Doing it themselves puts them in a position that's a little difficult to explain.