Not at all. Actus non facit reum nisi mens sit rea, and I agree with Hiddy, User Name and others than there is no evidence of mens rea in this instance. Theft is not a strict liability offence. Many people are not used to two-cabin aricraft or even assigned seats. I've been shouted at for queue-jumping when I've used the CE line (presumably the CE desk was taking people from the Economy line until I came along) and I'm sure that was an innocent mistake by someone who didn't even realize BA had two classes - if people can do that, they can innocently sit down in the first seats they see when told to hurry up.

I also agree that they crew were quite right to move them on, even if done a bit late. Mild embarrassment in front of a half-empty CE cabin is hardly a great humiliation anyway, is it?