He watches the bikes, I guess someone tried to steal my bike earlier and he chased the guy off.

Don't know why someone would want my 100$ Crappy Wally World bike, glad I will never bring the hoss here. So probally gonna pick him up coffee gift certificate to Tim Hortons later. Was talking to him and I guess he has chased 14 people off in the last 3 weeks from here.

Ah well, glad they keep a look out. Hear so much of bikes being stolen and people not doing anything. It is not technically securities Job to watch bikes, but they put a camera up.

Why would someone steal it? Because they can. A $100 bike is an upgrade for some people. You can help the guard by sending a formal Letter of Commendation to his boss. Just do not mention the bikes unles the guard tells you watching the bikes is part of his official duties.

Why didn't he arrest him?

A) it's entirely possible he doesn't have the authority (though citizens' arrest would be an option)
B) It's highly likely the guy wasn't doing anything wrong (yet) , just bending over very close to the bikes and examining them for weak spots.

Fair enough.