It is an iterated game, though. It's socially beneficial for you to be seen behaving altruistically on national television. I think in most cases this is more valuable than whatever prize the TV show offers.
In that way it is an iterated game, true, but I'd strongly disagree that it's worth much being seen to be altruistic, and certainly less than the prize of ~£10-20k.
Being altruistic might mean something for those who know you, but will be drowned out by how you actually interact with them. Being on the show in any capacity is most of the benefit, and being memorable within that, most likely by being ruthless and not altruistic, is going to achieve the biggest impact.
