Problem is that when you buy software you often get more code than you paid for, and that code is hidden by a feature flag. Reverse engineering the code and fixing that feature flag is therefore illegal, because otherwise that business model wouldn't work. In practice nobody will sue you as an individual for doing that, but lets say that Microsoft does it to avoid millions in licensing fees, then you have a reasonable legal case against them.
If what you say would be completely legal then many big companies would absolutely start doing that. Do you think that would be a good thing?
It's easy to compile (or transpile in interpreted languages) the code needed to implement a feature flag entirely out of the binary you distribute to customers who haven't paid for it. I don't see that this is a problem.
If I buy a car containing eg. heated seats, but the feature is disabled, because I didnt select the proper addon package, who is going to sue me, if I add a switch, and connect the heating elements to 12V supply myself? Or buy an addon controller? Or even buy addon seat heaters?
In practice, if you buy the car hypothetically containing heating seats but the feature is disabled, then the hypothetical company (that is probably Tesla) will void your warranty and refuse repairs.
Which is a great argument for Right to Repair, but makes everything a hassle on the consumer side - any future interaction with the company around the vehicle is a landmmine waiting to be triggered.
That's certainly the companies' problem. If they don't want code to end up at the customer they can easily remove it from compilation. Not doing that is just laziness.
If what you say would be completely legal then many big companies would absolutely start doing that. Do you think that would be a good thing?