The difference is you’re not causing any damage to your body when you eat spicy food unlike hitting your finger with a hammer.
True, the capsaicin itself doesn't harm your body, but the pain can in itself cause some slight damage (and like any drug, an overdose of capsaicin can be very dangerous).
I’ve also heard of folks having esophageal tears due to severe vomiting.
All ‘reactions to’, not ‘reactions from’ I guess?
That said, capsaicin is recognized as a chemical agent used in warfare, and having been through some training, it does suck to be sprayed.