I mean, when it came to that 2010 bill, how many mangaka were for it and how many were against it, exactly? Ken Akamatsu ran for office and was voted in on the campaign of protecting the freedom of expression in art (his profession) that had been under attack for years before and after 2010. I agree adult material shouldn't ever be distributed to minors in any capacity, and if it was just limited to that in holding sellers/distributors accountable, I don't think there would've been much of a pushback against it. However, it's likely it failed to pass because unless every adult material is tagged/chipped/registered in some database that would trigger a fine/response every time a minor bought it
in cash, it's practically impossible to enforce, and you can't have police stationed at each bookstore. I also know that had that bill been put into full effect that a lot of mangaka could've had their manga/serialized magazines sold in less stores depending on the area, and so they may have been trying to protect their livelihoods. Amazon and online retail still hadn't fully caught on in Japan then, and even then, what if those online retailers chose to pull listings over the bill so they don't get in trouble? It's ultimately on the parents to ensure their children don't get exposed to adult material before they're of age, but you know these parent groups rarely ever self-reflect on how
they're raising their children.
There's a reason
Shimoneta was written back in 2012, it was to criticize these kinds of bills and parental groups (the same ones that Go Nagai lampooned back in the '60s) with something as ridiculous-sounding as "Law for Public Order and Morals in Healthy Child-Raising" which meant so much as Googling, reading an
uncensored (gasp!) anatomy book, or uttering such obscene words like "sex", "penis", "vagina", et. cetera
even in the privacy of your home meant getting an electric shock from a shock collar everyone has to wear and repeat offenders get vanned. In the series, this made it so a rising generation has no idea what the concept of sex and childbearing even is, let alone have an actual understanding of how puberty and hormones work, all in the name of helicopter parents protecting their little angels' innocence. This is shown in the actions of Anna Nishikinomiya where she's developing romantic and sexual feelings but has no idea what's happening to her body and her mind because her mother was the very mastermind behind this law, and she goes legit rapey-yandere from it. It's played both for laughs and for horror as you watch her legit molest and sexually harass the main character without any form of self-awareness about what she's doing.
Hilariously enough, in proving the creator right, parents actively complained about the anime whenever they caught their kids (teenagers, most likely) watching it instead of studying/going to bed even though no nipples were shown in the broadcast (I believe even on the AT-X broadcast) until the end credits of the final episode. Meanwhile I have no idea if these same parents complained about
Qwaser of Stigmata from five years prior over its lewd breast milk fetish and a lesbian(?) S&M gothic loli, but I'm guessing not since that show aired at 3:30 AM on Sundays while
Shimoneta aired before midnight on Saturdays, and
Qwaser was published in the same monthly seinen magazine the second half of
Grappler Baki was serialized in.
Manga and anime have gotten popular because the freedom of art allowed creators to get weird and wild if they so chose to (hence the doujin scene). Sometimes it's because that's legit the artist's fetish, and sometimes it's for shock value or for storytelling. I think it's only gotten to be a problem in recent years because of the Internet, and young people have been finding it harder and harder to separate fiction from reality, though this is shown to be
also true among the older generations because Grandma accidently typed in the wrong word or clicked on a link without a second thought. On top of that, Japan's (post-war) taboo against sex of any kind meant the government can't keep up with the times, so you're always hearing of censorship bills being introduced but never taking full effect, and it's considered a safe bet to assume it takes about ten years before Western social issues start showing up in Japan for such pushbacks to occur. If anyone is old enough to remember if there were similar things happening here in the States around the 2000s involving media censorship or the redefining of obscenity laws (I don't think it was just about the violent video games), I'd appreciate if you help clear this up for me.
But it's more likely the real issue is we're looking at it through the eyes of Westerners with no clue on how the Japanese government works, we just hear "attacks on freedom of speech/expression" and think it's similar to how it works here.

All I know is that in recent years, the same "no fun allowed" fuckheads from here have been infiltrating Japanese committees and censoring material coming out here because boobs and lack of black/gay people in the cast offend them, and the Japanese are noticing. It's quite telling that no woke/woke-adjacent anime has made a big splash over there, like you're not hearing of Japanese fans throwing a hissy fit over how
Wonder Egg Priority ended like you have with the trannies here for instance.