It's true that lots of Utilitarianisms have corner cases where they support action that would normally considered awful. But most of them involve highly hypothetical scenarios that seldom happen, such as convicting an innocent man to please a mob.

The problem with LW/SIAI is that the moral monstrosities they support are much more actionable. Today, there are dozens of companies working on AI research. LW/SIAI believes that their work will be of infinite negative utility if they are successful before Eliezer invents FAI theory and he convinces them that he's not a crackpot. The fate of not just human civilization, but all of galactic civilization is at stake.

So, if any of them looks likely to be successful, such as scheduling a press conference to announce a breakthrough, then it's straightforward to see what SI/LW thinks you should do about that. Actually, given the utilities involved, a more proactive strategy may be justified, if you know what I mean.

I'm pretty sure this is going to evolve into an evil terrorist organization, and would have done so already if the population weren't so nerdy and pacifistic to begin with.

And yes, there are the occasional bits of cautionary principles on LW. But they are contradicted and overwhelmed by "shut up and calculate", which says trust your arithmetic utilitarian calculus and not your ugh fields.