I needed to order some cells this morning and remembered this thread. Here's a little research. The intent of the regulations seem to be to prevent short circuits and external damage, so I assume the explosion danger comes from overheating due to a short.

I think there are three situations: battery installed in a device, battery in retail packaging, and battery in other packaging. The first two are probably cool, but if you have a couple of cells loose in your pocket with your keys and change, you might have more problems than just getting through security. "Dangerous" means a certain amount of lithium or a larger amount of lithium alloy - I don't know how much lithium is in a 123A.

There are a couple of governing documents. For the US see
49 CFR 173.185
Elsewhere look for
United Nations Recommendations on the Transport of Dangerous Goods, Manual of Tests and Criteria
I would assume they are similar but wouldn't bet on it. The CFR does refrence UN documents for testing.

And this Quantus link has it broken down in simple terms.

Can you buy lithium cells in the airport? Next time I'm there I will look in the camera supplies.