That means you have the following options:
a) deport nobody, i.e. you don't apply the law
b) deport just the parents. What do you do with the minor children? Separating them from their parents (different countries) would be cruel.
c) deport the entire family, including the US minors. Since they have US citizenship, they can always return to the US.
Having deportation as an actual threat, reduces the amount of people who attempt to break the rules since they know there are consequences.