But it's quicker to make a shandy than to do a half a lager and half a lemonade. By that reasoning a shandy should be cheaper than two halves. (which it isn't)
It is in the UK cos there's a great law about it. In other places it's often near 80% of the pint cost for a half, which is a crazy rip and encourages drinking more.
Edit. Googling this I can't find that it's actually a law, must be just a very good historical precedent
But it's quicker to make a shandy than to do a half a lager and half a lemonade. By that reasoning a shandy should be cheaper than two halves. (which it isn't)