Airbags and pretensioners operate by a heating element acting on pellets of Sodium Azide which very very quickly produces Nitrogen gas to either inflate the bag or operate the pretensioner
Whilst not explosive in nature the violence caused by the rapid expansion can cause a lot of damage.
The other problem is the Sodium Azide itself I've posted on here about this before, it's an extremely hazardous material, if the airbag or pretensioner was damaged in transit then there's a danger that the Sodium Azide pellets could be released.
Even a small amount can make you very ill indeed and can kill as it's easily absorbed through the skin especially if it gets wet.
So be careful if you're climbing around in scrapyards if the vehicle is heavily damaged!
That's why they can be difficult to post, lots of firms just won't take them.
Saying that I received one I bought from eBay it came via Hermes, don't know if they told them what was in the package though
Russ