I think its not mandatory because originally it was so cost prohibitive, even if its not quite as bad now. If it was mandatory, less people would do trackdays and that wouldn't be good for business.
To be honest, i've been to many trackdays and there are rarely incidents. A track like Bedford is actually even safer because its so wide and open with plenty of run off. The last big incident i saw was at Brands when a CSL owner with too big an ego went off at Paddock because he didn't want to let a faster car past. If you're that kind of person, get insurance!
I agree with what people are saying about the marshalls, he should have been off the track after his first warning. I would put some accountability with the TDO.