They're really doing it based on the reason they think the PM is doing it. That he's up to no good. But the Queen understands what he's up to and approved it anyway. She knows. The court knows too. They overruled the monarch because they don;t like what's up. She has no power.
Alan, there seems to be something about the English legal system which persistently confuses you, even when you are given entirely correct information. John's formulation was exact. The Queen acts - by convention, must act - on the advice of her ministers. The advice she was given was to prorogue parliament; and so that is what she did. It's a fiction, of course, that the prorogation was her decision, but it is, in our governmental setup, a useful fiction.
In England*, a very strong Divisional Court held that the issue was not justiciable and therefore no investigation was possible. In Scotland, the Inner House of the Court of Session held that it was justiciable and, after investigation, that the PM's motive in giving the advice he did was, in its words, to "stymie parliament".
It's not clear that the Scottish decision has any effect south of the border. Technically, it could be argued that the Inner House, being equivalent in Scotland to the Court of Appeal in England, is a higher court than the Divisional Court in England, from whose decisions in non-criminal matters appeals lie to the Court of Appeal. However, given the composition of the particular Divisional Court (it comprised the Lord Chief Justice, the Master of the Rolls and the President of the Queen's Bench Division of the High Court), that point is somewhat moot.
The matter will reappear before the Supreme Court (which is supreme in England, Scotland and Northern Ireland) next week; the Claimant in the English case has been given permission to "leap-frog" from the Divisional Court straight to the Supreme Court. Then, we'll see.
The whole thing strikes me as an entirely futile waste of time, but it's made a lot of people awfully excited.
Jeremy
*"England" in this context should be read as "England and Wales"