If it was switched on, why haven't they shown us the images? I can only presume that it was switched off. Hawkeye is now an integral part of the game, and the match shouldn't have kicked off it it was not in operation. If Hawkeye was switched off that implies gross negligence.
The ball was clearly over the line. The players knew it, the goalkeeper knew it and the Sky commentary team knew it. The ref probably knew it too, but couldn't give a goal without getting the Hawkeye confirmation on his watch.
That's where this system differs from VAR. It's not about opinions at all. It's black and white. The ball was either over the line, or it wasn't. If we didn't have Hawkeye, the ref could well have given the goal. But he was forced to rely on Hawkeye and was not allowed to overrule the technology, which he beleived had ruled it was not a goal.
If Hawkeye wasn't turned on, then it didn't overrule the ref. No decision was made. If the ref knew Hawkeye was turned off, he would be within his rights to give a goal. But the ref thought it was functioning, and the lack of an alert meant the ball hadn't crossed the line.
If it was a thousand-to-one error the technology can be forgiven. After all, refs make poor decisions all the time, and even VAR doesn't always get things right. But if it was turned off, that's something else. Don't the PL rules say that Hawkeye must be turned on before a game can start?
If it was turned off you could argue that the game should be void. It would be akin to starting a game with one goal smaller than the other one, or the penalty area lines marked wrongly.