Jast have officially gone on record as saying it's a legal requirement, not licensing. They, like Mangagamer, Johren, and others, IP block their sites because they have uncensored images on it. If they wanted to, they could sell in Japan -- indeed some of the games these companies have the rights to are sold in Japan on Steam, it's just the adult patches that remain IP blocked (even when those patches are compliant with Japanese law). Again, the reason being that the site the patches are hosted on is not compliant with Japanese law.

The law is outdated and hard to enforce with any entity outside Japan. Jast only IP block because they don't want their Japanese partners to be indirectly accused of breaking Japanese law.

But given how social media makes access to uncensored porn in Japan so easy, this law really should be abolished. But doing that would deny a golden parachute to aged politicians.

Now, Crunchy Roll, on the other hand, IP block their site because they're hiding something. They even block news articles that are about events in the Japanese market, which would indicate they're not doing it for legal reasons. But then, CR has a rep in Japan among some in the industry as being highly suspect.
I don't put any stock in what Japanese companies claim. They want to sell to the Japanese market? Yet intentionally keep prices high to the point of strangling their own market reach. They even go so far as to recall all unsold stock. Brand new, sealed, never touched, perfect condition stock and they pay another company to destroy it. They don't even consider reducing the price to attract the audience share that generally waits to buy cheaper second hand copies. Nope. They'd rather lose profit by paying to destroy their stock.

The vast majority of my eroge collection comes from this unsold stock. My wife works at one of the companies that is used to destroy it. And for a while, employees at that company were allowed to "rescue" items as long as they never sold them.

That was until an employee was caught selling the stock. They were fired, the rules were tightened. Now lots of perfect condition games are destroyed without a chance of being placed into the hands of someone who would appreciate them.

Valve don't even understand Japanese law. They are breaking it every day simply by letting people buy CERO Z rated games with Steam wallet funds. How do Valve get away with it? Because CERO isn't technically the correct ratings board for Japanese PC games. That would be ECOS. And if Valve recognised ECOS, they could sell far more eroge on the platform in Japan. But they don't. And yet, there are plenty of games (from non-Japanese publishers) sold uncensored on Steam which are most certainly in violation of the indecency law regardless of which rating board the publisher claims they submitted to. Valve don't actually seek proof of submission, which too, is a grey area, and part of the reason CERO got pissy with Ubisoft when AC Valhalla was released.

The PC version of AC Valhalla is uncensored and was never submitted to CERO. As such, displaying the CERO rating was in violation of CERO regulations -- companies have to pay a significant amount to CERO for ratings.

So yeah, when it comes to ratings, and the law in Japan, Valve are clueless.