Because the US uses both. Metric is not particularly common outside of certain things (ie 100m dash, etc) but most educated people in the US know how many meters in a kilometer, etc. Whereas you have people in this thread who have no concept of how many feet are in a mile.
If you want to really confuse people from the US, display weights in grams. Then you will get an equivalent reaction.
I suspect that if there was some sort of mechanic in which you had to measure weight, and it was a matter of grams or kilograms, people still would have managed to grasp it without much issue. Weight goes up, number goes up. Weight goes down, number goes down.
As an Aussie I'd like to say the toilets flush the wrong way too. Though they nailed the murderous wildlife.
That gave me a laugh. I had a conversation with 3 Assuies over Thanksgiving (They were visiting the US) and they brought up the murderous wildlife as well. Also talked about Austrian Bachelorette and how overly complicated American football is.