Illegal to say illegal

I really wish it was illegal to say something was illegal when it wasn't. Some pubs do it when they don't want to serve certain drinks that look wrong to the untutored eye like a Black and Tan or snakebite. Instead of being honest with the customer and saying "the landlord won't let us" or "it is against the pub company policy" they lie about the law thinking people are less likely to argue.
It happened to a woman in front of me in the supermarket the other day, I don't know if it was bad staff training that meant the checkout staff didn't realise the difference between the law on the sale of Paracetamol and painkillers in general, or perhaps he just thought ibuprofen contained paracetamol, or it is a badly implemented a company policy but still.
I suppose it would just be another law that people objected to you pointing out to them (oh the irony) given they generally go into utterly nasty defensive mode when you call them on their mistake/lie and given how shitty people are when you point out they are breaking the rules themselves; this mostly happens with traffic regulations and the smoking ban, not had to say "excuse me, do you know holding up a building society with a shotgun is against the law" in a while.