I coordinate volunteers for the V8 Supercars events. They receive lunch and polo shirts which costs a fortune. If we were to pay them - V8 Supercars would be out of business.
By your definition, we also would have no State Emergency Service, Volunteer Marine Rescue, Rural Fire Service... or we would - and your rates and taxes would at least triple.
Volunteer work where nobody is being exploited is perfectly acceptable. It's also a great way for people wanting to enter a chosen field to get some experience (where nobody will hire them without it) and for many people to 'give something back' to the community.
Granted this isn't the case with extras for a commercial - but if the participants are willing... (and who wouldn't enjoy some ERT and free entry for the day)... then what's the harm?
Or is it your argument that there are other people out there who ARE paid extras, who are missing out on a days pay because DW is using members of the public?