I get what you're saying, but I also don't think it something they need to address as soon as dirt starts becoming visible on the snow either. "Dirty snow" isn't a bad thing in this case as long as it still looks like dirty snow you would expect to see in real life as well, and I think the current look falls within that range (although I hope I have forgotten what dirty snow looks like in a few years, for now it is still a fairly fresh image for me). It will need attention in the future (likely by next year), but I don't think its too the point we need to worry yet.