Check the money graphs. The Scipii might have a mountain of cash as Omanes says. Despite having a negative income they may be living off a cash stockpile from years ago when their empire was bigger.

Another explanation is that having hurt them bad they now have less army wages. When the Capua city income next recalculates it wont be negative anymore.

On VH campaign a faction gets a minimum of €10k income per turn. They still have to pay army wages though so theres a limit to what they can do.