The math is based around starting with a 15, give or take racial bonuses, and raising it by +2 at your earliest opportunity.
A feat is balanced to be equal to that +2. The ability to Power Attack and Cleave with a two-handed melee weapon is definitionally worth exactly +1 to hit and +1 to damage with that weapon. If you give any option for anyone to take both of those things, then that option will be over-powered relative to what the game expects.
Not that the game will be ruined, or anything, but it's very much like taking an array that has a 17 instead of a 15. It just is more powerful.