I can see why they want it; it's a great shot, and there are only daytime shots on the website, as far as I could see.
That 'proves' to me, they did not engage a professional photographer to 'sell' the site, as any photographer worth his salt, would have investigated the possibilities, explored the site, and would certainly have come up with some atmospheric evening shots.
Now they rely on a guest with an interest in photography, and a 'good eye.'
They should have offered you at least a 'money off' voucher, in my opinion.
If it had been my shot, I too would be pleased to be asked.