I believe the typical law regarding trespass is that you demand that they leave, and if they refuse, you call the cops. It's a misdemeanor offense, typically, whereas citizens arrest in Texas requires that you witness a felony in progress - and illegal entry is not automatically a felony.
(Or drug traffickers...or hey, illegals trafficking drugs)
Actually, that would be a felony in progress and actionable as citizens arrest - assuming that someone would want to take on a drug smuggler who is probably armed.
Hey, I'm with you - folks should be able to arrest someone in trespass. But the law aint' written that way, and these groups have too many political opponents just waiting for them to screw up to be dabbling with confronting illegals and patting them down. Best to observer, track, notify the Border Patrol and make good, solid publicity for the cause at hand.