It's harder to catch them.

It's pretty easy to comb through fiverr and pick off schmoes advertising fake reviews.

Identifying buyers is a different matter entirely.

I wouldn't be surprised if Amazon leveraged their position against the sellers to have them turn on the buyers.

I mean, how much loyalty is five dollars going to buy?

What is their relationship with Amazon? In most cases, they won't have any contractual relationship, which I would think makes it harder to obtain a cause of action. They could still sue for false advertising, but they'd have to show that the reviews contain statements of fact that are false. If I'm understanding correctly, this is a considerably higher bar than breach of a reviewer's contract with Amazon.

No, because then they might be taking on an entity with enough money to provide a competent legal defense against them.

s/efficiet/ethical/

I may be generalising but I believe most of the individuals targeted by the lawsuit are not in the position to just shrug off a fine and lawyer costs. But as Donzo said, it's certainly easier to sue them than the businesses that (IMO) are to blame for the fake review business. Maybe the goal is for the reviewers to turn over their accounts and thus the businesses involved?

This is exactly what they need to do. So long as there is demand there will be a supply. You need to kill demand to kill the market.