Yeah well I never see them get away with it. Spies are a bad intel capturing class. So are snipers. They just don't have the speed, health, or weaponry to get out of the base most of the time.
I get away with the intel as a spy all the time. I can usually get it at least close enough to my base for someone else to grab it and take it the rest of the way.
Besides, if I get in there and sap whatever sentry is guarding it, I don't want to stand around and wait for a scout to come by and take it for me.