The Boise Mall is private property. As such, they can post signs or simply ask you to leave. The individual stores are rented space and are treated as seperate private properties.
Failure to leave when asked (when signs are not posted) can result in an arrest, but usually the responding officers will only ticket you on misdemeanor trespassing charges. If you leave when asked (or leave to disarm when asked), no charge can be brought against you.
If it is posted and you enter, you can be held for trespassing. You will be arrested.