Toki Pona is simple enough that you don't need the book to learn the language. The books ("pu" and "ku") are not a prescriptive manual but more like a descriptive dictionary, and indeed, a recent census [1] shows that those books weren't the most used learning materials.
That was an issue with the language Loglan where the author claimed copyright on the language. As a reaction the community created Lojban. Basically GNU/Lojban.
Whether you really can copyright a language is pretty dubious though, probably not. In the case of Loglan it was not enforceable but Lojban is now far more popular anyway, so it is moot.
As for Toki Pona in practice it is not much of an issue because the free learning material is good enough. The language is very small and not that strictly defined anyway but I get your concern.
The community of toki pona speakers has grown to the point where there is no such thing as an authoritative source, much like a natural language. If you want to learn in depth your only option is to join a chatroom and talk to people. For intro material, there are many free options beyond the official book. https://devurandom.xyz/tokipona/ is quite good.
I know what toki pona is. Just like Delphi, you find little roadblocks. License problem here, can't legally give it to friend, can't put it on your website. These are all completely artificial problems that simply don't exist in free languages. That's why Delphi failed despite being superior at the time and that's why toki pona will fail.
You are knee deep in a cow piss on a remote farm in Nebraska, offline and some library has some version issue with your proprietary licensed language. A trivial solution if it was free language, but now it requires 300 miles trip. You are in Nagorno Karabakh and just spoke to some random stranger about this new funny speak-language but you cannot give him PDF because it's not free. These issues stings very deep because how artificial and easily solvable would they be. Some things must be free otherwise they cannot be used to their full potential. So you stand there, in Nebraska or Nagorno Karabakh and swear - Never again will I use proprietary language. I have nothing agains proprietary software or linguistic books but the language itself must be free.
> offline and some library has some version issue with your proprietary licensed language
How is this different from any proprietary book about a freely-licensed language?
If you're in a remote farm in Nebraska and you only have a copyrighted Esperanto textbook, you can't share it, even though Zamenhof's materials might be public domain.
I fail to see any problem with toki pona here.
Something similar can be argued about Delphi / Object Pascal. Borland's compiler is non-free, but the language can be used with FreePascal. I'm happily using FreePascal on my personal projects and haven't touched Delphi in more than a decade; it's a nice language and there's nothing wrong with it.
Sharing FreePascal is totally OK, just like sharing freely-licensed materials about toki pona.
IMO speaking TP is not restricted by licenses. i can speak it completely offline. and IMO it's designed primarily to be spoken, not written. so a paid or non-free book is not an obsticle, since TP's design philosophy wants to include everyone possible around the world (eliminating voiced/unvoiced plosive pairs, hard to pronounce vovwels, consonant clusters), so a latin based script does not belong to the language's core; there is even a Hangul script for TP.
Bit of a side-track, but if you ever get to go to Korea, spend a few hours learning how Hangul works. I am crap at learning how to speak languages (17 years in Germany, and I still have most people shifting to English as soon as they hear a few sentences from me), but I managed to learn how to use Hangul pretty much on the flight to Korea.
Helped immensely in getting around! Of course, I had absolutely no comprehension of what things meant, but I could match sounds to signs and avoid eating things I’m allergic to.
