Weak groups need to step on the heads of even weaker groups to gain influence. If anti-FBI-overreach activists appear to be aligned with pedophiles, that'll weaken the cause of the activists. Instead they should appear align themselves with some group that's more respected than themselves, like believers in the constitution. That's perhaps the reason pro-transsexual activists don't widely support Chelsea Manning or why bullied schoolkids aren't helped by the majority of the students who are all struggling to maintain their own image.

Not that it's a very nice thing to do, it's just self-interest.