So why don't you spit into a handkerchief or tissue that you have in your pocket? That is not an excuse to be rude and gross out others.
Well, I said I don't run inside for this particular reason (or, at least, that's one of the reasons). Outside, I make sure no one's around. As someone mentioned, finding running shorts with pockets is difficult. Also, a spitting into something generally doesn't work too well. Not to be gross, but a kleenex or even a handkerchief wouldn't be enough for a three mile run a lot of times. So I generally try to hit the grass so other pedestrians can't see it. I live in a secluded enough area and my running hours are such that this approach works about 90-95% of the time, and a lot of the time it doesn't I get caught by a fellow runner. And on the rare occasions I don't, I apologize and continue onward. It's gross, but it's not the end of the world.