Essentially what the current system advocates, is forcing players to play in a certain manner. This is where the argument lies.
Forcing someone to play without excersizing thier chosen playstyle, in fact, circumventing it entirely (IE: Not giving them the option), is bad form.
PvP in any sort of effectivity in the BGS is only usable in a defensive manner, you cannot win at the BGS attacking in such a manner, that is only effective in PvE. (Even then, you still need folks to be commiting transations to the cause to have a decent chance of defending the system)
That should not mean that PvP is permitted to be entirely avoided via sitting in PG/Solo mode whatsoever.
This is an equality issue, if someone chooses to use PvP to defend against someone attacking thier system, then they should be able to do so. Logging into solo/PG to prevent this is along the same lines as combat logging.
"I don't agree you should be able to shoot me, so I'm going to attack your faction from Solo/PG and force you to defend in a manner to which you find vapid and boring, don't want to play ball? Then best hope other members of your faction will, or we win" That is a very bias setup if I ever did see one.