Some FReepers will say Zimmerman didn't have that right.
And I would agree with them if Treyvon was walking down the sidewalk in the middle of town. But this was a gated community, not "public property". Gated communities by there nature are "private" usually run by a homeowners association and you are expected to abide by the rules if you are on the premises. Now was Zimmerman working "for" the homeowners association in his capacity as Neighborhood Watch Captain and therefore had the right to enforce the rules? I don't know. But if this goes to court, this is what it could hinge on. If the the Association rules say that paid security or volunteer neighborhood watch can challenge anybody they don't recognize, then Zimmerman is on pretty safe ground doing what he did (even though the 911 operator advised him against it).
Zim then got out of his truck/car and followed him on foot carrying a firearm which he then used to kill Martin.
So, anything that leads back to the theory that Zim could "challenge" any visitor is a dead end. That's because he actually didn't "challenge" him earlier. He challenged him "later", on foot, after he'd reported what he was doing to the 911 clerk who advised him to not follow him.
Not that Zimmerman had to obey the clerk, but the next legal issue up after that event is whether or not Martin had a reasonable fear for his life, and since he's dead, we'd have to say YES!
Really folks, keep in mind that this case has a dead body and whatever theories you come up with have to deal with that fact. So far no one has come up with any reason why the shooter had to shoot the victim!