I remember the first time I saw my husband walk up to the waitress and hand her the tip, I thought it was really strange. She also gave him a strange look and was a bit embarrased. It was almost as if he invaded some kind of personal space. Americans have this wierd philosphy not to discuss 3 things with people you don't know well: Politics, religion, & money. Perhaps directly handing the tip to the server violates that unwritten rule in non-verbal language. Which is why we leave it on the table and walk away. The server picks it up after we are gone.