I haven't heard this, but I very much doubt it's true.
However, the landlord is allowed to decide what happens in their pub, so I wouldn't be surprised if there was some arsey bastard that was opposed to it and decided to ban it and maybe everyone just took it as being the general rule?
I remember when I was living in East London and the council banned the Union Jack flags being displayed in the market as it could be seen as "offensive"
Abosulte bs, I was absolutely fuming.. and I was only 12