There are Roman citizens within the city. Nola is most notably not a city of the Samnites, or at least not any more. It is home to as many Romans as Samnites, and has been for centuries. Appius, for his part, is far more merciful than Tercerian, and suffers the people within to live. Many escaped when the Samnites took Nola, but those who did not live in subjugation.
You are already starving out Roman citizens as a necessity of war, but poisoning them or depriving them of water would probably see you in a hot spot of trouble, and could be spun into a case for your exile by a charismatic enemy (i.e any of them) in the future.