What changed wasn't the addition of curation but the constraints of it. The reason entire school libraries were emptied following the "book bans" is that these laws often use poorly defined language to classify what content and topics are permissible or not in school libraries and this means all existing literature has to be carefuly combed through to decide on a case-by-case basis whether it violates the law, especially when the consequence of an "illegal book" carries a fine or worse.

It's similar to the abortion bans: the problem isn't just that they ban abortions, the problem is that what an abortion is is often poorly defined because there are plenty of scenarios where a pregnancy has to be terminated to prevent harm to the pregnant person but we wouldn't normally think of this as "getting an abortion". The vague blanket bans mean medical professionals need to get a legal opinion on every individual case because they face liability if the procedure turns out to have been illegal in that situation (and not, for example, if they had performed it 24 hours later even if the progression was predictable at the time).

The same is also true for teaching sex ed in states which use vague language like "age appropriate" or blanket ban certain behavior outside a strictly cis-heterosexual norm (e.g. a teacher telling her students she got married to her husband likely won't get her fired despite her disclosing her sexual orientation whereas a gay teacher might not be allowed to disclose theirs).

Even if you think the state should decide which books can go in a public school library or not, certainly having a central register that reviews each book and classifies it is more efficient and more manageable than just making every librarian or school individually liable if their library carries a book deemed inappropriate after the fact. After all, review boards already exist for films and TV.

I'd chalk it up to typical legislative imprecision in writing laws... except for that imprecision's pervasiveness.

Now, I think inspiring self-censorship is exactly the intent.

It makes it easier to pass censorship laws ('We're really talking about {most egregious scenario}. We'd never charge someone for doing {something lesser}'), while simultaneously apportioning governments more power to selectively weapon the law via prosecutorial discretion ('It'd be a shame if you fought us on this. Everyone has done something wrong...').

On the one hand, in the case of book bans, creating an actual state-mandated review board to make these decisions in advance would make it much easier to call this out for the censorship that it is while offloading the decisions to librarians or schools and making them liable for guessing wrong frames it as a matter of "personal responsibility" and framing violations as parents making themselves heard instead of the state intervening directly.

On the other, it creates a culture of fear around "taboo" subjects ensuring that those that would be held liable for transgressions (the teachers, librarians and school boards in case of the book ban) will over-comply and apply the most far-reaching interpretation of the law in order to minimize risks of liability because even when you haven't actually violated the law you don't want to risk having to demonstrate this in court. This is of course done in full knowledge of the intended but unstated parts of the law, e.g. if "sexual orientation" becomes a taboo subject that only means "anything other than perfectly straight" but also the mere status of being in a homosexual relationship or being trans will be treated as taboo and a risk of liability (which in turn influences hiring decisions as the legal risk of hiring discrimination is lower than that of an emboldened activist parent suing over perceived immodesty).

And finally of course as you say it creates an almost blank cheque presumption of illegality that can be used by weaponized law enforcement through unequal application of the law. This is perfectly exemplified by Russian laws that don't outright make it illegal to be gay but make it illegal to "promote homosexuality" where you then you simply sue visibly or openly gay people for "promotion" instead. In the case of the anti-abortion laws this can be as simple as having had a miscarriage or a delayed period and being forced to prove your innocence.