It's Valve's website, what they say goes. Pedophilia should not be tolerated in any way. Honestly Valve should have had it to where they track this games purchases and report them to authorities. That being said, depends on how much you want controlled and regulated or not.
But that is a tricky thing to judge.Where do you cross the line between people that are phedophiles, and ones that simply like the cute girl asthetic, without having any sexual connotation to it?(The same reasoning would apply to games and animes)
As long as there is no explicit sexual imagery or heavy suggestions of pedophily on the game, I say its fair game.Otherwise its just Valve censoring what is essentially, in my view, different takes on a subject due to different cultures.
But one thing I will agree with you is that this is Valve service.If they dont want it because they dont like it, then its their right to remove it.