How about a libertarian argument?
If you want to get rid of the welfare state, go for it. But as long as we have one, as long as government is 'providing' (i.e. taking our money in taxes in exchange for programs we might, or might not want) for us, it's wrong to strip people of their rights as a precondition for using the service.
I wonder if you've considered the kind of precedents such laws set? You're suggesting that anyone who applies for welfare give up their freedom from unreasonable search and seizure.
What if we applied this concept to health care? Government is well on it's way to taking over health care. Will you be suggesting similar requirements for anyone "leeching" off taxpayer money in that arena? Should we be required to prove that we have maintained good personal health habits, or pay a penalty/tax if we can't?