Why are they allowed to take down sites so obviously unrelated?
I imagine it's pretty easy to use this sort of takedown in a malicious manner to remove competitors' sites, surely there is some defense mechanism against this?
It doesn't. This outfit, per Chilling Effect's search, pumps out notices by the hundreds. Most likely they use some script or tool to grab a bunch of URLs from various Google searches, then dump them into a form letter. I'm not sure if anyone was supposed to weed out the obviously bogus output, but I think we can all see that nothing of the sort happened.
They churn these out fast enough that nobody is actually paying any real attention to what they're doing from what I've seen.
Why are they allowed to take down sites so obviously unrelated?
I imagine it's pretty easy to use this sort of takedown in a malicious manner to remove competitors' sites, surely there is some defense mechanism against this?