Why? Because not all powers are created equally. Sure, if you're a combat pilot for a power that's been heavily involved in combat and attracts a ton of combat pilots, forcing everyone into open sounds like a fantastic idea.
Suppose instead that you're Torval, Sirius or Mahon - powers that pretty much have absolutely no interest for combat pilots - why are you so intent on ruining their ability to actively ruin the fun that their players can have? They have no pilots capable or even willing to defend them, because the game rules have actively discouraged combat pilots from joining them.
I didn't jump in this discussion because of Mahon, I support open Powerplay because it can create more variety in gameplay.
Your argument can be turned 180° : if the situation is like that is because the game rules - change the rules, the situation will change. Put merits for the same activities that produce credits (escorting cargos, killing players that have large bounties) and the scenario becomes profitable for everyone.
When I (Mahon pledge) fortify in open, I am taking a MUCH higher risk than any fortifier from Aisling, ALD, Patreus, Hudson, Winters or Sirius, because I have to bring my merits through a bottle neck. We all risk facing a rebuy screen, but Antal, Delaine, Mahon and Torval pilots fortifying are also risking a complete cargo load of fast tracked merits - which in my case works out to between 7 and 8 million credits.
Do you think that combat pilot in open have nothing to lose? I may be interdicted in any CS by an enemy wing and lose all merits in a heartbeat, eventually making that the system(s) I undermined won't be such. Obviously nothing like that happens because everyone is in solo to maximize their profits and minimize their risks.