We rely heavily on strangers out there, reading the stories, remembering other stories they've read, and getting suspicious enough about similarities to report it to us. We've caught a lot of plagiarists because of people like that. We're well aware that other sites don't seem to care all that much about plagiarists, but most of us are writers ourselves, and we know how we'd feel if our stories got plagiarised, so the least we can do is to ban them and publicly humiliate them. And we'll keep that up, as long as people keep bringing these things to our attention.
I'm glad your situation got sorted.