It's a surprisingly complex legal situation. It's very clear that using a domestic Sky subscription in a pub is a breach of copyright, but it's less clear if us

It's a surprisingly complex legal situation. It's very clear that using a domestic Sky subscription in a pub is a breach of copyright, but it's less clear if using a foreign satellite subscription is legal - recent court cases have gone both ways and involve both British and EU law. The fact that a pub is showing a football match without a suitable license from Sky isn't necessarily evidence of wrongdoing. The glass provides an instant clue for inspectors, but also the general public. A busybody (or a rival publican) can instantly spot if a pub is using a dodgy satellite feed. There's also the underlying tension between Sky Television and the English Premier League, who both have slightly different incentives around rights enforcement. The pint glass allows Sky to easily audit pub landlords, but it also allows the Premier League to audit Sky. It's not a comprehensive anti-piracy system in itself, but it's a really useful and really simple tool.