In Minnesota the law specifies a sign of size and type stating no firearms allowed. However, if you violate it, you can only be asked to leave the establishment. If you refuse, you can get a $25 petty misdemeanor ticket.
The law also prevents landlords from banning weapons. So, in a mall, the individual stores could do it but the sign at the entrance to the mall is worthless.
I carry all the time. The Mall of America is at the other end of a light rail line from Minneapolis, and there have been gang bangers shooting each other there.