It supports the developer by preventing blatant ripoffs of the Teensy.

Paul is one guy, and has put in a ton of effort writing high quality libraries for it. Most or all of them are open source. The main MCU is a commodity item. Only the bootloader chip is closed source.

If you want to rip off the Teensy, you can use the same MCU but you'll need to come up with your own bootloading process (Adafruit could do this if they wanted to). It wouldn't be that difficult but is enough of a barrier to stop casual cloning. Seeing as how Amazon and Aliexpress are filled with cheap Arduino clones but not Teensys, it seems to have gone well so far. Nobody wants to be undercut so easily by someone who has no intention of contributing back.

What's so special about the Teensy bootloader? If it's the only closed source component, why not just replace the bootloader with an open source one?