It is, the level of the pint glass changes every day, so the "enterprising publican" would have to send someone out everyday and change his sticker every day too. Sounds like quite a bit of effort.
And fundamentally, the visibility of the pint glass is not a "guilty / not guilty" switch. Even if something that looks like a pint glass is on display, no matter how well faked, it still comes down to whether Sky have a record of having sold a commercial subscription to the address at which it is observed.
So it's a useful indicator, but only one of many, and therefore doesn't have to be fully bulletproof.
