Because the Pilots Federation doesn't tolerate members destroying other members unless they're wanted. The PF has its own code and would naturally place bounties on members who violate it without regard for what a powerplay leader thought of it.
The code has plenty of exceptions dealing with just this scenario. Volume III, Section 7, subsection ii says:
Pilot's Federation Charter said:Federation and Imperial Loyalists
The Pilot's Federation is a Union of ship Commanders from many walks of life. We recognize that some are feel loyalty to a star nations, and may wish to actively support that nation, above and beyond what is outlined in this charter. In order to accomidate these Commanders, and to ensure the security and prosperity of all, we all parties involved in conflicts between the Powers that Be obey the following rules.
- All Powers must register with this Union to be recognized as such. The Power is required to insure all Union members against the loss if their ship, and pay the Union 1000 credits per day per Commander in administration fees.
- All Commanders wishing to directly support their Power must register their pledge if alliegence with the Union. All pledged Commanders must display their loyalty on the IFF systems at all times.
- A Commander is to be free to leave the service of a Power at any time. The Power may, at it's discression, seek restitution in the case if betrayal. (See also: II, 1, ix)
- The Union agrees not to sanction armed conflict between pledged Commanders. Any losses incurred by these Commanders is insured by their respective Powers, and not by the Union itself. As such it will judged a private matter, and no further actions will be taken.
Addendum a) The Pilot's Federation recognizes the Alliance as a Star Nation
Addendum b) Due to the growing number of Powers and sub powers, the Pilot's Federation will acknowledge only 12 Galactic Powers.