It's the same no? If there are 10 times more fake reviews than legit ones and they can spread them over time, you can't use the distribution to detect it.
I think there are probably a number of reasons it's different.
1) 1000 5-star reviews with maybe a handful of 3,2,1 star reviews trickling in as actual consumers realize their mistake is less of a signal not to buy than if I see another low review trickled in amongst the 10-20 5-star reviews.
2) The company defrauding by purchasing the reviews is not going to make their money back as quickly probably since they won't end up on top of search results as often. So these companies will be stretched a lot thinner and probably recoup their expenses a lot slower than if they start instantly showing up on top of peoples' search results. And in order to keep from "floating" a bunch of free stuff without reviews they'll have to be extremely coordinated about how they distribute their products and request recipients to provide the reviews in order to make it a worth while pursuit.
