Feck! :-o
Hmm, that exclamation works on several levels...
Legal or not, I think there should be some sort of contentious objection clause when it comes to declining a particular job. That's not to say one should be able to get all silly and refuse anything that comes along on the ground you can't be arsed to work or anything.
However, categories of work that some people may find a direct offront to their personal dignity/safety or morally unnaceptable need to be reasonably considered. Simply saying "humping complete strangers for cash is legal, so do it or we stop any benefits" is absurd IMHO.