Some people living in places that have become tourist areas are putting up signs announcing their home toilets are not for public use. Because apparently some tourists have said things like "When I needed to use the bathroom and there was nowhere else around, I knocked on a random person's door and they were kind enough to let me use it!" So now a non-zero number of people go there with the expectation that they can (and possibly should) do the same.
Tourists used to be a novelty to Japanese. Now with over 40 million projected for this year, a massive rise from about 6 million in 2012, a large number of them taking extended vacations (in contrast to Euros who might hop a border for a weekend and boost tourist counts quickly), people are getting quickly burnt out with the entitlement many of them exhibit. To tourists, it's a magical, unique vacation and they must have the Ghibli experience someone else posted about. To locals, countless people are harassing you everyday demanding unreasonable things.