That's true but they run a scheme where ST ticket seats can be resold to members if they can't make it and they quite often don't sell them (ironically, the illegal ones do sell because they're part business deals or they sell to tourists). Arsenal have various membership schemes.
I wasn't even eligible to buy a Reading ticket even though I've been a ST holder since moving back to Cardiff in 2011. It's a f*cking disgrace tbh.