Keys on G2A should usually be genuine.
The problem is that these genuine keys can be obtained illegitimately, by using stolen credit cards. Once a key has been purchased using a stolen credit card, some developers/retailers have had trouble matching the invalid payment to the key they've released. I'm not sure if this includes Steam.
The result is worse than piracy, as the credit card charge back actually costs money and you're giving your money to a criminal.
Meanwhile, the thief gets away with a free genuine key, and can resell this somewhere like on G2A.
My (potentially naive) point of view is that it's actually pretty stupid if a retailer is unable to link a transaction to a specific key in order to have the key flagged as not genuine once illegal activity is identified. That said, if it's Steam's fault, then Larian shouldn't have to suffer for it.
There will also be legitimate keys on sale on G2A from people who contributed to the kickstarter but got more keys than they needed.
