It’s a warranty & legal thing. Prusa had users flash unsupported firmwares, break their electronics with bad configs and then claim warranty on them, and there are also ones available that have safety features tweaked or disabled that could end up with a fire.

You are still free to do it, the tab just a permanent way of clicking “I understand doing this voids the warranty and I’m going it on my own risk” - which you’ve basically always been agreeing to modifying the firmware anyway.