DJI has proprietary firmware in their battery. Disallowing 3rd party firmware or supplies is a form of DRM and is legal. HP does it with their ink cartridges. And while it's not the same thing, Keurig does it with the K-Cups....
I honestly do not believe that DJI would be so stupid as to create a blocking code in FW to prevent use of other batterys - that would be illegal in many countries. Even Microsoft had to stop doing it with Windows and their exploitation of the Browser wars.
The browser analogy is invalid because the bundling of IE in Windows without a means of easily changing the default browser was stifling competition with other browsers. If DJI's DRM strategy was to stifle competition with other drone venders, you would have a point.
If you do a Google search on "Non-DJI Intelligent battery detected. Aircraft flight prohibited", you'll get a lot of hits for where the drone owner was blocked from flying their drone. You'll find hits on this site and links to videos like this one
I have no experience with the Powerextra batteries and I take your word that they have worked flawlessly for you. But I don't want to be blocked from using an app because DJI detected a dodgy firmware.