Why can you treat a bee sting with ammonia?

You can treat any sting with a plethora of remedies, the main reason most of them work is placebo.

The toxin a bee injects is a combination of formic acid and some much more complex proteinic toxins.

Ammonia is basic and the idea is that it will neutralise the formic acid in the sting (which was until recently believed to be the primary cause for pain) however, as soon as the bee's stinger is removed, the already tiny hole starts to close so it's unlikely you'd even get ammonia into the injection site by pouring it.

It is also important to note that Melittin is now recognised as the most powerful pain-causing agent in bee stings so logically, a good, safe remedy would be plain aspirin, as it inhibits the primary inflammatory mechanism of the venom (COX).