On The Side, On The Front, But NEVER On The Back

My sister requested I put this on the blog, because she really, really, really wants to know the answer:

Earlier today we were at the National Portrait Gallery (my favorite of the Smithsonians, by far). Sis was wearing a backpack. The guard stopped her and told her she could wear her backpack either on her side, or on her front, but she could not wear it on her back.

Sis said "okay," and started walking around the museum with the backpack slung over one shoulder, like we all used to do in 1997. On the side, right? But one room later, another guard stopped her and said that too much of her backpack was on her back.

"On your side or on your front," he said again.

Sis carried the backpack in her arms for the rest of our visit.

Sure, rules are rules, but there were all kinds of women walking around the museum with those giant tote-purses worn over one shoulder and resting against their back. Why weren't they being stopped?

And what's the big deal about wearing something on your back anyway? Our best guess was that maybe if you wore something on your back, someone could sneak something in, like... a portrait.

Anyone know why they came up with this rule? (And why totes are exempt?)