It also depends on whether it is a private vs. public college.
If it is a private college, they can stipulate a zero-tolerance rule to tobacco in the dorms, and enforce it.
Private institutions are not limited by the law in the same manner as public institutions. At a private college or university, a student's rights re: his/her privacy in a residence hall are probably spelled out in the rental agreement or some other document. These limitations will likely vary a great deal from one campus to another, as some schools believe strongly in allowing students as much independence as possible, while others are more aggressive in making sure students are obeying the schoold disciplinary rules.