I think that it was completely called for. As a GCSE student who took the paper a few days ago, and as a student who frequently gets As and A*s in both papers, I looked at this question and simply didn't understand.
The reason I found it so difficult was due to the wording and the information given. Normally, the question would at least give a value that we can logically work with, even for an A* question but this didn't give us anything.
Everyone's arguement against the petition seems to be that there is no way that the exam board would set such a hard question if pupils were unable to solve it. Well tell me this, if it was appropriate to GCSE, then why did it just come up on BBC news that there has been an uproar on social media because it was so hard, and that 10 000+ pupils have signed a petition?