Why is it Illegal to Deface Money?

Ever thought of defecating (to use the technical term) on your leaders?
Put some paper money on the floor and do it. It’s so much fun.
Stupidly many countries prosecute you for this. Don’t they realise that defaced money is a donation to the treasury? Once defaced, money can be printed to replace it.
FREE MONEY FOR THE GOVERNMENT!
Every government in the world should be campaigning for people to go out into the street and defecate on the image of their leader printed on money.
I do hope this takes off.