I think you may have gotten that wrong. Airbags are worse if the seatbelt is not worn. The timing for it firing takes into account that a seatbelt restrains the forward movement of the person. No seatbelt means moving forward quicker which means getting the full brunt of the airbag. Best case whiplash, otherwise a broken neck or similar.

At least that is what I have learned and which sounds reasonable.

I dimly seem to remember that there might be differing strategies in place in some parts of the world (maybe something like US vs. Europe?): In some places seatbelts aren't assumed in determining the airbag timings, whereas elsewhere the timing and triggering level are optimised based on the assumption that seatbelts are being used.

I don't think your point and their point are mutually exclusive:

- no seatbelt, you want the airbag to catch you (better than no airbag)

- with a seatbelt, ideally the airbag doesn't (need to?) go off, but if it is necessary you'll be glad of it

If there's blood, I think it's fair to say they should have gone off. I'd have those airbags checked and possibly replaced.

> They’re primarily intended to provide some protection for unbelted passengers

I don't think they're meant for unbelted passengers; they should be belted first. I once saw a program about the difference between regular airbags and airbags designed to be used without a seatbelt, and the latter were enormous monstrosities that could knock you out. They were determined to be a bad idea. (I wish I knew where I saw this, but it was decades ago and I don't.)

Your head can still hit the dash if you have a seatbelt. It holds your body, not your head. And then there's things like side airbags, etc.

Supplementary is in that it supplements your main restraint - the seat belt.

They should always go off if the front of the car is damaged as the description here.