I think coming right out and calling them modesty shorts is a bit of a marketing fail. They're just screaming out for people to get pissed off at the concept that has been much discussed on here already.
As a concept, they're fine. I can see lots of people buying them for the same reason they buy skorts. Cosy, comfortable and useful.
I do feel that by dressing girls and boys differently at school - skirts instead of trousers, flimsy Mary Janes instead of sturdy boots - Clarks, I'm looking at you - we are already teaching girls that they should be less active than boys, and behave "modestly" and appropriately.
"Modesty" shorts will not address this. Unisex, practical school uniforms/dress codes that encourage/permit unrestricted movement and running around would, though.