Can Japanese citizens freely go back & forth to the US right now on leisure trips?
I think it largely depends on the state you're in. Unless the Japanese government is limiting where citizens can fly to, they're probably able to fly into most major American cities. However, that doesn't mean there aren't draconian restrictions once they arrive and Japanese being obedient and overly-cautious by nature they're probably smart enough to stay away.
My current state has very, very strict rules and that's why we're enjoying almost zero new cases per day. Returning residents considered non-essential are required to self-quarantine at home for 14 days, which most are happy to do since nothing is open, anyway, and they can stay home from work on paid leave. All "visitors" (i.e., tourists) are warned NOT to come here, and if they do they are required to sign a contract promising to self-quarantine in their hotel rooms or places of residence for 14 days. Of course, none of them actually plan to abide by this so vigilant hotel staff and volunteer groups monitoring social media are alerting police and violators are arrested, charged by the AG's office, confined in solitary and then shipped home ASAP. Since tourists stick out like sore thumbs to begin with, cops are interrogating anyone who looks like a tourist for ID and proof of residency or quarantine fulfillment and if not, they're arrested, charged and sent home. Anyone who arrives and can't show proof or either residence or accommodation (homeless) are arrested, charged, kept in isolation and then sent home. I'm in favor of shutting down all non-resident entry as are most folks around here but we have a particularly dickless governor and mayor.
I hate to say it but other than the gym being closed, I'm really enjoying this lockdown. Traffic is non-existent, almost zero tourists around acting like idiot fucking tourists, I'm saving a shit-ton of cash just staying at home drinking on my balcony or rooftop. I'm hoping they'll stop all tourist travel and re-open for residents and it's going to be fucking awesome.