That's been the law for some time on coaches, and hao been in place longer on the continent. Having said that, for people over the age of 14, it's their responsibility to wear their seat belt. Presumably for younger people, making reasonable adjustments, announcements and relying on parental supervision also helps. It's hard to enforce, but common sense says that you should wear a belt where provided on a bus. I have seen a minor incident on a school bus that could have been prevented had they had their seat belt on.
As has been mentioned before, road vehicles are totally different from trains. Trains are on a safe track, with plenty of protective measures. So, before you even set off, you're considerably safer on a train than on the road, where even the best driver can be involved in an accident just being in the wrong place at the wrong time.