The same could be said for any issue on parenting from school to other extra-curricular activities (piano lessons for example). I don't think it's an acceptable excuse. It costs nothing to run outdoors. And it costs nothing to do push-ups and situps on the floor. It may not be as fun as going to a gym to work out but it is effective. And hell if they aren't eating enough then aren't likely to be overweight anyway.... However, when it comes to stuff like this, I'd also support a universal breakfast and lunch programs so that kids can be fed right as well. But we'll save that for another discussion.....
Have a look at the standards for the IB. Aside from the academic standards there are significant components of community service, creative development (music, art, etc) and of course physical activity. That's a well rounded education. Requiring one credit in PE in grade 9 is not. If you don't want a test, then we could require kids to play x amount of hours in vigorous sports. That would probably achieve the same result. I'd rather set a standard and then let kids get there on their own (personal training, sports, group training, etc.). Perhaps give them tests every year just so they know where they are (does not account towards anything).
Back on topic....I am surprised that prevention is not discussed when it comes to the US. Obesity rates there are exploding too. And I am sure all this stuff is probably starting to impact their economic competitiveness.