To be fair there are more and more devices on the market that are made to assist cheaters.[1][2][3] Banning third party controllers will even the playing field for all players of competitive games. Besides Microsoft obviously wants to monopolize their overly expensive controllers. Microsoft themselves have already made Adaptive Controller which isn't too expensive and supports a wide range of external devices. You can think what you want about the ban but you can't frame it as them banning accessibility devices specifically.

[1]: https://arstechnica.com/gaming/2021/07/cheat-maker-brags-of-...

[2]: https://www.cronusmax.com/

[3]: https://www.consoletuner.com/products/titan-two/