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I will let 1 of our resident experts answer this,
just wanted to welcome you on board.
Shak
Have you actually tried entering a URL as a keyword, or you going on the circumstantial evidence that of the URL's you tried, no adwords appeared?
I suspect however that a valid URL has been detected and the query not sent through the usual query processing routes. If you enter a URL that is indexed you get a specific page of information about that URL, and I'd wager that this takes place before the Adwords engine is reached.