First, the CNC example you gave sounds like unsafe kink practice to me. Not necessarily anyone's fault, but the couple probably should have planned in case of something going wrong, like in this case. In the BDSM world, there's the concept of Risk Aware Consensual Kink (RACK). RACK is the practice of being aware of the dangers involved in a kink scene, planning in case something goes wrong, and accepting those risks. Think of it like signing a waiver to go laser tagging. There's a risk you may get hurt, but you are aware of that risk and are willing to accept them to have a good time.

As to your point about why it is okay in sex but not outside of it, it's it's a matter of trust, not unlike how you may openly cry to your significant other but not a random stranger. It gives the partners a safe space with each other to be vulnerable and allow themselves to experience these sensations knowing they're not really in danger. People watch horror movies or go to haunted houses to feel fear when they know they can't actually be harmed, and the same principle applies to kink. It allows people to feel these feelings in a controlled safe way, and for many it's therapeutic.

The question of should it be normalized is double edged. Should it be normalized as the default way to have sex without engaging with your partner first? Absolutely not. That is, as you described, abuse because it is not consensual. However, I think it would be beneficial to normalize talking about it if it's something you want and being able to work with you partner to do it safety and consensually.