PowerPlay affects the game world for essentially all players. Each power applies their effects on exploited/controlled systems, and, if one plays in Open, one is going to encounter other CMDRs who are either performing business on behalf of their power, or are utilizing equipment that could only have been obtained by pledging to a power.
My CMDR has never been pledged to any power, but he regularly leverages their discounts, and has occasionally supported or opposed CMDRs engaging in PP activities in order to help preserve or repeal a power's effects (most of my CMDR's high value CMDR bounties were collected by destroying PP merit farmers who thought that because my CMDR was not pledged that he wouldn't mind losing his 15% LYR discount in the systems he haunts). He's also been at shot by every PowerPlay weapon in existence, though some considerably more than others.
The main impacts of Powers' presence for most are beneficial (asides from PP ship spam which ive said elsewhere in this thread would be a win-win if removed, as would benefit Powerplay gameplay if they were removed & merits rebalanced accordingly, as well as to reduce irritation for non-PP partakers) Ship & module discounts as you say in LYR space, increased bounties for BH in my power's territory, mining bonues elsewhere etc. All these are benefits for pretty much everyone.
My point though, was that random players affect the BGS, but dont affect Powerplay. But yes they do benefit from it for the most part, so yes are affected
by it. But thats not the same thing regards the marketing slogan about all players in all modes affecting the BGS in the shared universe. Because that doesnt affect Powerplay because already, all players dont affect Powerplay unless they opt-in by pledging. Hence, theres no legal basis for a challenge if Powerplay was made Open Only. tbf, i much prefer Rubbernuke's variant where activities are split between modes. This doesnt exclude anyone from the feature, but also doesnt persist with the blind assumption that all modes are equal, when they simply arent in practice.