Smoking culture in Japan appears to discourage smoking outside. In many places smoking outside is illegal. However some trains, many shopping centers and most restaurants have designated smoking areas.

As such you’re somewhat less likely to see cigarette butts on the street in Japan in any case.

You know those little smoking enclosures that some airports have?

Yeah, those are all over Japan outside (albeit not air-tight ones). You only see people smoking in them or in buildings where smoking is allowed, commonly bars.

It's a pretty great way of controlling cigarette butt litter, actually.

Yup. Also, when smoking outside, people do have a strong incentive to dump the butt ASAP, whether there's a trash can in sight or not - cigarette butts are dirty, smell really bad (even for smokers' standards), and can foil clothing.