But I can't see any logic in the directives stopping applying once you play First Class...
Well, logically speaking, you are absolutely right. But try telling a FC team that they can only bowl their new fast bowling talent for 6 overs at a time. If they were to do it, it would certainly bias selection chances for someone like Bresnan. Besides, he IS the exception rather than the rule and for most youngsters, this situation just would not apply so I guess it shows just how hard it is for a rule to apply evenly to everyone.
Of course, his county would have to be conscious of his long-term health (I mean, if he is THAT good they would want to look after him to ensure they get the best out of him), they would not bowl him too often anyway.