I've never understood why it makes an ass out of the "me" in that context. I can only imagine scenarios in which the person doing the assuming is making an ass out of themselves, not where their assuming is making an ass out of other people.

Some people simply respond more strongly to clever wordplay (Ooh look! It's the same letters!) than to sound reasoning.

Because assumptions generally make fools of both sides - you'll assume that someone will tell you and they'll assume that you knew.