Drug testing for some welfare recipients in Pennsylvania

I don't really understand the constitutional objections over privacy to drug testing. Someone doesn't have to take government money, but if they do there can easily be certain requirements, including a drug test. From Fox News:

After a federal judge blocked a much broader drug-test rule in Florida, Pennsylvania is taking a more careful approach. Instead of mandating drug tests for all welfare recipients, Pennsylvania plans to randomly test only those with a felony drug conviction within the past five years and those on probation for such offenses.
Officials are taking it slow. A pilot program has started in Pennsylvania's Schuylkill County, which could pave the way for a statewide program this summer if it proves cost effective.
State Sen. David Argall said in a statement last month that the program is "overdue," as officials try to cut costs in the state's most expensive division -- the Department of Public Welfare.
"This initiative seeks to stop the abuse within our welfare system," he said, adding that government benefits should only go to those "who genuinely deserve state assistance." . . . .