Their reason is: people is using g2a for “discounted” keys.
Where the “discount” comes? Easy, some asshole buys from their website many keys with a stolen credit card, then they will need to refund it + pay an expensive fee for the chargeback.
I’m not a dev but at that point I would just give up selling keys by myself and I would just rely on steam for fraud detection. The only case where the 30% fee is justified