You can probably blame the litigation mentality on that. Schools have had their insurance premiums bumped up as a result of that and it is an ever increasing upward spiral.
I had more than a dozen broken bones playing rugby league in my time at school. This was mostly during the lunchtime stuff when we all played very rough. I, my parents and just about everyone else at the time, just accepted this as normal schoolyard boisterousness and no-one would have ever tried to apportion blame on the school. Nowadays, parents would be suing the school or the State government for negligence.
I've heard of some schools asking parents to sign waivers or to seek contributions to their insurance costs. Rugby league is the big casualty of this, but they are not on their lonesome. I understand that the situation is much the same for rugby union and aussie rules. I'm sure that it won't be long before soccer and basketball go down the same path.
We'll end up with a generation of fat, lazy kids who would rather play with their Gameboy, Playstation etc than get off their ample backsides and kick a footy around in the fresh air. Perhaps a bit of a sweeping generalisation, but I've got a quite a few American friends whose children attend schools where contact sports are effectively banned, and we seem to be heading down that road.