Because the incentive is now there. Maybe they don't get enough paid customers and want more money. This puts a bit of pressure to move a new feature that is really handy into the paid level. Then another and another. Might not happen but it could.
Most people using Datastar will not necessarily be smart enough to fork it and add their own changes. And when Datastar makes a new release of the base/free code people will want to keep up to date. That means individuals have to figure out how to integrate their already done changes into the new code and keep that going. It's not a matter of if something breaks your custom code but when.
Finally, many people internalize time as money with projects like this. They're spending many hours learning to use the framework. They don't want to have the effort made useless when something (ex: costs or features) changes outside of their control. Their time learning to use the code is what they "paid" for the software. Doesn't matter if it's rational to you if it is to them.
I'm only working in local dev right now, so i've got the pro version and inspector going. When I get to prod, perhaps this will be a problem.
Yet, surely, this could just be toggled with an env var or db setting or something? if dev, include pro and inspector component. If prod, use free version (or custom bundle that only has what you need)