Why is G2A disapproved of?

I recently discovered the existence of G2A, and I saw some really low prices on certain games.
However, searching on these forums I found many people advising against buying such keys if you want to support the developer, I also found that some keys sold by this site have been revoked.
Are the keys not originally bought from steam or the developer? Why should this damage the developer if he was paid for those keys?
Moreover, why should steam revoke such keys? Is there no protection of the customer?