Some packets of meat flavoured crisps are suitable for vegetarians so why wouldn't they eat them? Seems strange, although I suppose if it is against their principles then who is anyone to argue? I can see where they are coming from.
I had a Google and it looks like cheese and onion (Walkers, anyway) are suitable for vegetarians (not sure about vegans). Their 'mature cheddar' flavour, however, is not.