Same with PayPal. Bottom line, you agreed to a TOS. The fee from those transactions keeps PayPal running.
For digital items on PayPal, you really don't have any protection as a seller, which is why so many people request F&F. There is an inherent risk in these sort of transactions.
An established, reputable seller faces the risk of a hit-and-run chargeback from a scamming buyer in non-F&F transactions. Conversely, if you're buying from an established, reputable seller, there is very little chance they'll screw you as it would affect their reputation, especially on a community-based site like this.
If they're not well-known, then sure, you face a risk buying from an unknown. It's totally understandable to be wary of paying F&F in a situation like that.
But that is most frequently not an issue on CAG.
As far as the fees, I'm willing to bet these sort of transactions comprise a tiny, tiny percentage of PayPal's revenue. Keep in mind that this sort of thing didn't exist on a large scale ~8 or so years back, but PayPal has been a juggernaut since the turn of the millenium. eBay is what keeps PayPal running, not our $0.50 Steam game sales.
If PayPal offered any real sort of seller protection it wouldn't be an issue. Screw their TOS.