good video on the subject: YouTube - Broadcast Yourself.
makes a good point about how league 1 and 2 clubs are allowed terracing yet championship/PL clubs arnt which seems a bit strange.
Pretty sure that lower league clubs were exempted on the basis that:
(a) most lower league clubs rarely have crowds anywhere near capacity
(b) the cost of making their grounds all-seater would put a lot of lower league clubs out of business