I grew up in NYC seeing old drunks do this on the subway (think AquaLung) and skidding on pools of spit everywhere... so anyone blowing their snot out on the ground in plain view of others, or where people might chance to walk will always seem a gross and disgusting display of lack of manners to me. Heck, you might as well just squat and poop while you're at it!
Guess I'm just hopelessly 'old fashioned'!