Many, many malls, including outdoor malls, have the policies people have mentioned, about no teens under a certain age left to roam on their own. Remind her that malls and stores are private property - not public property; there is no "right" to be in any mall or store. Too many teens and their parents forget that fact and forget that any mall or store can order anyone to leave.
I think you made the right call. An outdoor mall (what I'd call a "strip mall") generally does not have a lot of security, and when it does -- like one near our house -- it usually indicates there have been issues there in the past, as at the one near our house and some others I know of.
"Hanging out" in spaces like malls and stores is asking for trouble and isn't a creative use of kids' time. I've had friends who found that letting kids have too much free rein on hanging out at places like malls etc. ended up with the kids meeting up with other kids the parents didn't want them hanging with; or the kids would leave the property and "just go next door to the other shops" etc. without understanding why that would be a problem in the adults' minds. Malls are for shopping, which can be fun, but not for hours of endless hangout. At 13, that can happen at home or the kids can come up with something else better to do.