well, on the one hand, there's nothing wrong with taking pictures in public places. what's more, sharing the pictures by customers on social network might even help the mall itself with promotion.
however, on the other hand, malls are private property, and guards may prohibit customers from taking pictures there for some reasons like:
a. protecting privacy of other customers;
b. maybe they suspect that images may be used for planning smth illegal like theft;
c. maybe they think that taking pictures makes other customers uncomfortable;
d. maybe they just have a bad day;