I think I've heard this two or three times in various places now. Do bar staff get training to refuse pints of some beers?
The first was upon requesting a pint of Leffe in the Adelphi in Leeds. I settled for two halves and walked away rolling my eyes.
Most recently, I was in Port St and requested a pint of Little Evelyn (a Marble/North Bar collaboration I think) which was 7%. Hardly stupidly strong! I decided just to settle on a half this time rather than order 2 of them. A shame, as there were a few there that I fancied trying but being told that I can not have a full pint of a beer I decided to head off after the swift half. There's a reason people order a pint rather than a half and being told you can only have a half just serves to wind people up.