Drug tests are unconstitutional without probable cause. But since, all drug warriors hate the Constitution with a passion, this is not a problem for them.
You take the King’s coin, you do the King’s bidding.
It's an effective process to weed out (no pun intended) illegal drug use, and it's a condition of service. You've never been in the United States Armed Forces, have you?
If service to my country requires I provide a specimen at random intervals, I see nothing wrong with a recipient of government funds (i.e. "My Money") being required to do the same as a condition for receiving said funds.
I figure, if someone else is paying your way... You have nothing to say, except perhaps if they are deliberately doing things to make it hard for you to become independent.
To paraphrase Franklin, it is not wise to make people comfortable in their poverty.