I know what you mean, obviously mixing fast with slow is a recipe for disaster and it pressures new drivers to go beyond their limit... BUT, if they did split the groups it would be a whole lot more work and admin (most likely needing to book before hand so they can sort numbers, or sort them on the day which would take a while and need extra staff) which would put the price up of the day which is part of why it's so great.
There aren't really any major accidents and probably no more than any other track day. When you're at a bike track day at philip island they state that stats mean that one in 10 people will crash on the day. It's been true so far that I've been attending with all safer methods included and the $250 price tag.
If people could follow the rules then it would be all good really... but they don't. Oh well!
On another note, I'm currently watching Speedweek at Winton, they're going super fast!