The PL are involved because the fines apply to promoted clubs who are then members of the Premier League and no longer members of the Football League. The FL therefore need the co-operation of the PL to impose fines.
There is, as I understand it, no windfall for clubs that adhere to the rules. I believe the Premier League and/or their sponsors, baulked at the idea of money that should go to Premier teams being paid to league teams. That was the original idea by the FL but was never ratified by the Premier...
http://www.financialfairplay.co.uk/financial-fair-play-explained.php
Forest are taking a gamble. If it pays off and they get promoted, they get a fine. If it fails then they get a transfer embargo and will probably kiss poisoned scottish dwarf goodbye.