I once asked whether they had coffee in a pub, then pointed out that they did not pay alcohol duty on their coffee, so wouldn't be entitled to a seat or to use the toilets - all the facilities paid for by the drinkers.
Absolute nonsense of course - but as were the road tax claims
That analogy doesn't really work as pubs are private businesses, so it is not taxes that go into the upkeep of the facilities but the profits from the drinkers, alcoholic or otherwise. If you want to use the toilet in a pub in passing and don't intend to stop to purchase anything it is polite to ask permission first.