It's no surprise it's selling so well. People are always looking to save money and if you're undiscriminating about what you drink then you'll part with a pound for an average pint and a penny piece back.
I'm amused though at stories of people adding blackcurrant to their pints. This is presumably evidence of not liking the stuff in the first place and buying it merely because it is cheap. Surely that's a step too far. Save money if you like on substandard products, but don't buy the stuff if you don't like it. That's just ridiculous.