Here's why. Tom proposes that group selection produces pressures for individuals to engage in elaborate forms of signalling so that they can secure themselves as a member of that group. Crucially, this needs to be irrational, so that groups maintain their stability over evolutionary time-scales and are allowed to be selected for. The premise, of course, is that human social groups are a good way for individuals (or gene-lineages) to propagate themselves downstream.
This premise is the one which doesn't stack up, and is generally regarded with skepticism in evolutionary biology. Why? Because from the perspective of an individual (or their genes), adaptive fitness would be much higher if you exploited a group, than if you did your best to be a co-operative member of a group. This point is most vivid if we consider a possible proto-human social group: the hunting party.
It seems to make sense for a collective of cavemen to co-ordinate hunting activity, since it reduces the costs to any individual hunter, while allowing them to track bigger game (woolly mammoth et al) and thus maximise their payoff. But consider any individual hunter. They could do even better than the others, if they simply slacked off their hunting duties but still had access to the spoils. Of course, this kind of reasoning is symmetric, so every individual will think the same way and try to free-ride off the group. Obviously, this will destroy the stability of the group in favour of individual fitness.
The simple way to think of it is this. If the group is thought to be the unit of selection, then the challenge is to show that successful groups don't simply grow in number, but actually spawn new groups which are similar to themselves - groups that are their offspring. Otherwise, the best explanation for human groupish tendencies isn't group selection, but simply a cascade of individual selective pressure. Pace Tom, but I don't think his account paints the correct picture of the evolutionary architecture of political commitment, because it doesn't establish how these groups get going in the first place.
However, all is not lost for the conscientious Darwinist. I still agree with Tom's conclusion, but with a slightly different flavour. The answer, I think, is a question of cultural rather than biological evolution. And not just in a metaphorical sense, but in the literal application of natural selection to the propagation of social information. This is a big topic in itself, and hopefully one which I will explore shortly.