I do like the idea of half a pint = half the price but I can understand why it shouldn't be. Basic economies of scale, when you go into a pub you are paying for;
a bartender, a (clean)glass, a stool, heating, lighting, possibly security, possibly entertainment and an amount of beer.
For comparison, an 8oz steak is always more than half the price of a 16oz and no one ever complains.