I don't think preposition is correct. It's still being used as a subordinating conjunction with the second clause being an implied complete sentence. Take the

I don't think preposition is correct. It's still being used as a subordinating conjunction with the second clause being an implied complete sentence. Take the current top comment example. "because politics" actually implies a sentence something like "The talks failed because of politics, which is the kind of thing that always happens when politics are involved." It's a common rule-check for grammar to substitute these complete sentences in order to understand the short-hand rule applications.