> Some places like Okinawa and Kamakura beach are generally cleaned
As for Okinawa that seems to depend on the island. When I was there some years ago, many beaches on the smaller islands were covered in trash (mostly of Western origin – one could often still decipher the letters on e.g. beer and shampoo bottles).
That doesn't necessarily discount it being local waste due to the large number of foreigners on Okinawa (American military, mainly) who get familiar products shipped in from abroad. The military base even has its own mall that locals can apply to visit and buy American products.
Personally, I'm not surprised Japan has a plastic waste problem, the shops and manufacturers here are addicted to plastic, and the culture is one of appearances above truth so I'm not surprised to hear about the differing levels of care dependent on whether tourists frequent a place.
I've also lived in other parts of Asia and I've seen some shocking things to do with waste. India was the worst. As another comment mentions, it's probably best that developed nations export knowledge and attitude towards waste, not just waste.
As for Okinawa that seems to depend on the island. When I was there some years ago, many beaches on the smaller islands were covered in trash (mostly of Western origin – one could often still decipher the letters on e.g. beer and shampoo bottles).