sure they can. but why would they choose to?

Because the language doesn't prevent them, and they own their library.

That's not really an argument. What prevents the author of a library in any language from acting in bad faith and use antipatterns? That's not a problem that would only happen in the Zig language.

the question remains – why would they choose to?

Because they decided to do so, regardless of people like yourself thinking they are wrong.

They can, and they wouldn't be necessarily be wrong.

However if the library is trying to be as idiomatic / general purpose / good citizen as possible, then they should strongly consider not doing that and only use the user provided allocator (unless there is a clear and documented reason why that is not the case).

I don't think it would make sense to restrict this at the language level. As a developer it's up to you to exercise your judgement when you examine what libraries you choose to depend on.

I appreciate the fact it's a common design pattern in Zig libraries and I also appreciate the fact I'm not forced to do it if I don't want to. If it matters to me then I'll consider libraries that are designed that way, if it does not matter to me then I can consider libraries that do not support this.

and so why should he be forced to not do so? he can cook his own thing outside of what most people using this language will do, and they will just not use it, and nothing's wrong with that.