I'm 'intelligent enough' to usually be able to work out which product offers the best value - even though, as several pps have already said, it's not always easy when some packets show a price per 100g, then another different sized packet of the same thing shows a price per unit (such as per biscuit) on its shelf label.
However, it irritates me beyond belief that I should have to spend so much time performing mental arithmetic almost every time I shop (depending on which supermarket it is and how transparent its pricing is) - I mean why, it's hardly customer-friendly is it - yet they'll often effectively have a captive audience because it may be the only supermarket for miles and people either don't want to travel further afield - or can't afford to use non-essential fuel, or don't have private transport.
And then I get really angry when I think about the many people who can't work stuff out as readily as I can, as Balloonslayer points out - and who undoubtedly will therefore be losing out by purchasing stuff (especially non-perishables) which is needlessly costing them extra, like Fluffy's mum. Particularly because people who find on-the-spot maths difficult will often be vulnerable - I don't mean to generalise but elderly people for example, or those with learning difficulties.
I can't think of ANY sensible reason why supermarkets don't ensure their labelling/pricing is clear and easy to understand by all customers. You can therefore only assume that so much of it is arse about face - albeit that they may be following the letter of the law but not its spirit, as a pp pointed out - because they hope, or indeed know, that a good proportion of customers will opt for the worst value item because it's advertised as a 'bargain', a 'special offer' or simply because they make the natural and not too unreasonable assumption that larger has historically tended to mean better value.
Makes my blood boil that 'we' are all but taken for fools.
Oh - and for those who've criticised people who 'fuss' and 'bother' about such things, get real, get in the real world and perhaps show a bit of sympathy for people who have no choice but to follow an ultra strict budget - for whom transparent pricing is therefore particularly important.