The police arrested the mom, not CPS. There was no interaction between the family and CPS prior to the arrest. The arrest is what brings CPS in to the picture. Just by virtue of arresting a parent, CPS should to get involved, because arresting the parent potentially puts the kids at risk... and then you double down on that because they've been arrested for endangering the kids. CPS then needs to figure out the lay of the land, so that they can establish that the kids are NOT at risk and they can step out of the picture.
Which is another way of saying that it is just as much a precaution as it is a punishment.
CPS responsibilities are to the children and their safety. The police & courts deal with the parents, collect any evidence, mete out any punishment, etc.