This is not entirely true. I've known several foreigners that worked in Japanese porn, as male actors. And the evidence is as clear as day, as the Japanese porn movies they are in.
Some porn companies will hire foreigners, it's a depends situation.
The issue is mostly the VISA and money. You need to be a permanent resident or be under a marriage VISA. Then you can work in porn. Both of these causes complication for the foreign guy. If you are married, getting into porn is clearly a problem. And it takes many years and is very difficult to become a permanent resident of Japan. To get to that point, most foreigners have normal jobs, so don't need to do porn. Porn for men, doesn't pay well, unless you are doing a lot of movies. And even then, your popularity can fade quickly (within 1 or 2 years), so you have to move on to something else.
You can't do porn if you have a regular work visa or tourist visa. You and the company that hired you can get in a lot if trouble. This includes the SOFA type visa that American military and employees are under as well, and there has been cases of those guys getting in trouble for doing porn too.
