It's time for a bit of a history lesson. Only 29 games have ever been given the official ESRB rating of AO (Adults Only 18+) in North America. This rating is of course not to be confused with M (Mature 17+), which is an entirely different (and less controversial) rating. Sony, Microsoft, and Nintendo do not allow AO rated games to be released on their console platforms, so this unusual Amazon glitch that happened recently is quite amusing.
If you visit the official Amazon storepage for the Nintendo Switch Pro Controller, you will see that the item is currently listed with the rating "Adults Only". This will likely be the only time Nintendo will directly sell an AO rated product, even if it's just an error. Check the page for yourself to step into a weird alternate reality!