If they just moved their servers outside of Japan, such problems would disappear for them. I seriously doubt if the enforcement authorities are going to verify that people's credit card purchases are not being used to D/L uncensored porn in Japan.
You're looking at it at the wrong end. The authorities aren't going to investigate
your credit card purchases, making sure you aren't
buying uncensored JAV. They investigate distributors' sales, to make sure they aren't
selling uncensored JAV.
From the ever-knowing Wiki (emphasis mine):
... under Article 175 of the Criminal Code of Japan, people who sell or distribute obscene materials can be punished by fines or imprisonment ...
For instance, if you wanted to sell JAV in Japan (eg: operating an office in Japan to handle all the related matters), you would need to go through various business registration, which would include authorities making sure you're not selling uncensored material. This is why if you've seen any number of uncensored JAVs, they always begin with a statement noting the location of the records holding and management office, with an address located (almost always) in the US -- even if the uncensored JAV was actually produced in Japan (which almost all of them are). You could of course sell uncensored under the table (and it happens quite a lot, so it would seem), but you would run the risk of getting caught.
But not every JAV production outfit is able to (or willing to) set up shop outside of Japan in order to circumvent Japanese restrictions on mosaic censorship -- and in many cases it's probably a lot of trouble for very little profit (thanks to piracy, like here on AO).
As for where the servers are located, there doesn't seem to be a completely black-and-white answer. Just as American servers are used to enable piracy of copyrighted Japanese content (think music, anime, etc) since Japanese lawyers seem uninterested in pursuing international piracy, I'm sure there's plenty of uncensored adult material floating around on Japanese servers for the same (or similar) reasons. Since P2P sharing doesn't really define a "buyer" and "seller", there's a lot of grey area in the above legalese. The only significant legal action concerning the internet has been against piracy (just like in America); but so far there doesn't seem to be any real precedence concerning availability of uncensored adult material in Japan.