I think the laws have changed in the past few years. It used to be that if you purchased in the district you had to register it. When I first moved to Dc I tried to register it at a police station and they had no idea what I was talking about.

Now, however, I don’t think you need to register it and it’s lawful to carry for self defense only.

https://code.dccouncil.gov/us/dc/council/code/sections/7-2502.13

https://mpdc.dc.gov/page/security-personnel-guide-self-defense-sprays

It’s possible Amazon hasn’t updated their restrictions to match current laws.

Here’s Sabre’s website with outdated info

https://www.sabrered.com/blogstate-laws-for-ordering-pepper-spray-online/