Nothing pisses me off more than stepping in someones big, bubbly wad of spit. It's disgusting, it's rude and pretty classless.
However, criminalizing it is about as stupid as it gets.
Back in my crawling around in the attic heat and air days it was against my law to spit in the attic. There is nothing nastier than crawling across some joists with a hand full of insulation stuck to your hand with brown spit.