I guess that's the catch, the employee regulations for both full time and temp agencies need to be equal. Something like a temp agency has to pay for 35 hrs a week regardless of the amount of work they have contracted. Then the temp agency can't have a limitless disposable labor pool at their beckoning. Or something like that to even up the costs.
You do not need to go that far, all you need is to make sure that temp and full time worker both receive the same benefit and pay the same expense (workers comp, unemployment insurance, social security, medicare, etc) by the hours.
There is need for flexibility in businesses especially seasonal work like retail or farming related.