"Should colleges be able to ban concealed weapons when state law allows it?"
The question is rather broad. Are we talking about private colleges, state colleges?
If it is a private college, some would argue the college has a Right to ban firearms.
If it is a state college, some would argue that the college can, and should abide by, and utilize all state laws at their disposal.
Students ought to be permitted to carry on-campus. If the state permits it but the college says no, I would still carry but that's just me--I would just CC on-campus.