This is nonsense, there are plenty of public trash cans in Tokyo at least. Not on every corner like in the States, but within 2-3 blocks you can find one and plenty of establishments have no problem letting you use their trash can.
Zero litter at the event, but yeah, some people are hella irresponsible w/ disposing of the trash after they've left the event. They're not just throwing it on the ground (that I've seen, anyway), but they're leaving it in any random trash can they can find, rather than paying to dump it (which there is ample opportunity to do -- a cottage industry springs up of people taking trash for $5 a bag post BM).
Probably burning man should just include end-of-event trash disposal in the ticket price, and have an official collection area people can dump their trash. (Probably some reluctance on BM part to do this 'cuz of the somewhat-inaccurate "radical self-reliance" principle.)