I wonder if this is because it has become a more visible thing to do. With TV we see sports people spitting all the time.
Children who used to go mostly to friends and school but now go to so many places on a daily basis are also seeing this all over the place as they travel to various places.

Its like swearing on the radio..... it happens more and social rules have lapsed... and children follow what they see and hear. So each generation lapses a little and changes what is socially acceptable...

(goodness, I sound OLD!!! oh, wait, maybe I am )