Where is this attitude of entitlement coming from? Are people entitled to use Nginx's code for free? Yes, free software is a wonderful thing, and I do believe that most software should be free. But the reality, in this world, is that software developers need to eat, and relying solely on donations only works for a handful of projects (as does relying on selling support). Open-core is a reasonable middle ground.
If somebody wants to reimplement that feature on top of Nginx and release it under a free license, there's absolutely nothing stopping them. If you want to use an already written and well-integrated version from Nginx themselves, then you can pay up.
Certainly someone could write and release a third-party, open-source nginx module to do bot throttling, if they wanted to.
Are people not entitled to make $300 a month because they owe Nginx for a bot throttling feature? Can you run a small website without throttling these days?