It's so simple. A big company has work it needs doing - so it EMPLOYS somebody.. And PAYS them NMW.. I was working at M and S, WHS and MacDonalds when I was in sixth form..These companies have been doing it forever.. they shouldn't participate in workfare schemes. Just give people jobs that need doing.
Yes, limited unpaid work experience is useful in sectors where it is hard to get the skills needed eg. you want to be a vet and need to work with animals, but come off it.. these companies can train up their new recruits for the kind of work they are offering in under a day.