Most adults also use mobile OS which are needlessly restrictive in terms of productivity software, even compared to Ubuntu, so it can't be just that or we would put the kids on Android and iPhone, don't all/99% adults use them?
EDIT: also how do you not think that Ubuntu using kids won't go on to use this free tooling in the future? MS spends a ton to target the education market because they know that you can get kids hooked early you get easier wins.