The argument goes thus,
Depending on where the client is you may have been travelling for hours in grid locked roads or rubbish public transport so it's right that your company covers your lunch because not only that but you have to get back as well.
It's also not your normal working day and you are putting yourself out for the company so in return they should at the very least cover your lunch.
Again this is all dependent on circumstances but it's good practise not to piss off your employee's by being tighter than a fishes arse.