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.
Supporting free and open source technology means supporting whoever can get it in as many hands as possible as cheaply as possible while paying your employees fairly. Adafruit gets this.
Making anything proprietary at this point for use in DIY software or hardware projects just makes me roll my eyes and hold my money for the day the open clones come. I have been saddened by dead open hardware projects because old versions of Teensy they were built around are no longer produced.
I will never buy a Teensy, but I look forward to buying the Freensy.
Sparkfun blew it by making the Teensy partially closed and so Teensy will be irrelevant. LibreOffice, OpenTofu, MariaDB etc. The most open solution always wins in the end. It is why I buy Prusa over Bambu, and why I pick Adafruit over Sparkfun.
My own companies likewise freely OSI license everything we do to the public no matter what the profit minded folks in our universe have to say. I -love- when people compete with us using our own work. If you can make a budget version of what we invent for people that cannot afford a bit extra to support us, wonderful. That is success for us as a creators that want to maximize impact.
If you cannot afford our prices, by all means buy from a competitor and contribute to the ecosystem that our customers benefit from anyway. That is why we will never spend a second of our engineering time creating or promoting proprietary technology because all the skills we have are because others shared their work with us.
I honestly take a lot of inspiration from companies like Adafruit as a leader of my own orgs.