It's probably to speed up the border crossings for the excursion.
I wonder if there's a non-Schengen version or the border crossings make it not worth doing.
I think it's the other way round, MSC Divina is due in Malaga tomorrow so the excursion would be from Spain to Gibraltar.
And not available for UK passport holders either.
Hope you can find another option in Malaga @Blackjackboss.