You mentioned below that this wasn't a policy of the store as a whole, but something that particular store did every so often.

I've been to this particular location and I have an idea as to why they might want to discourage tweens, teens, and younger kids from wandering around without their parents/guardians. The store is right next door to a movie theater. They've probably had problems with kids hanging out in the store before/after movies and causing a ruckus.

I have no real opinion either way concerning the employee's actions or your reactions, but just wanted to offer an opinion as to why this particular branch would have a policy like this when it wasn't a nation-wide rule.