User blog comment:TsurugiFan16/MSPD: Inazuma Debaters!/@comment-4880952-20130509175309/@comment-4893771-20130510153010

There could be multiple reasons for that:

1. Fifth sector was also forcing some children to do the training so he could ave been forced

2. Shuu lost his sister because he wasn't strong enough to protect her (according to him). And it was his ideology that only the strong are worthy of playing soccer so he took the training.

3. Fifth Sector could have threatened to destroy the island and as it's protector he allowed them to do their experiments.