If two people hand in the same essay in school, how do you know who actually wrote it? It is a judgment.A judgment based on things like writing style analysis, which is pretty easy. I have blogs steal my posts wholesale pretty often, and most people stealing your writing do a really shitty job of it.
I am not talking about what you think is fine or not fine I am talking about what is possible and actually happening.
What is actually happening is that so far nobody gives a damn about writing enough to prevent blatant infringement. It's not an incredibly hard problem. Youtube is solving a much harder problem with their audio/video copyright issues, and while their solution isn't perfect it's certainly stopped me from using them to reliably find movies and songs. The music I do find there is the sort of stuff that's not popular enough for the band to care. If they did, those videos would be gone within hours.
Remember that there's a copyright on text just as there is on anything else. Nobody enforces their copyrights because so far there aren't particularly huge rip-offs that are costing writers money, but if somebody ever does create anything that's effectively leeching then suing him would be ridiculously easy, since on the Internet everything is archived and most things are stored with dates. You couldn't possibly steal one of my blog posts and get away with claiming it's yours, and so if I care enough to go after you you have no defense and have clearly broken the law.