It doesn't matter that the % of seats that are RR is low, and the reason is because there are no capacity controls.
A RR ticket that was not sold and used on a half-empty flight is an obvious win-win for SWA and the customer, but when that same RR ticket can be used to occupy the last seat on an LAX-BWI flight, SWA is out $300 if someone else wanted to buy a ticket. SWA is OK with that because it doesn't happen too often.
However, with Ebay sales, you have people on those half-empty flights using a RR ticket in which revenue that would have gone to SWA goes to the seller instead (assuming they needed to fly anyway, which I assume is quite common with the typical ebay RR customer). Adding that to the RR tickets occupying the last seat, and the negative revenue impact RR has to SWA is something to be reckoned with.
Ticket brokers and privately arranged sales (not available on FlyerTalk of course
), while much more difficult for SWA to control, are also not nearly as easy to use as ebay ==> fewer RR tickets being sold and thus not as much revenue loss.