In general an insurance company is allowed to consider all resident operators of an insured vehicle, including children that are away at college since their permanent addresses is usually still their parents residence. This is because the insurer is using classifications that reflect a possible exposure for liability, in event that bodily injury or property damage occurs due to your child's operation of the vehicle.
So in essence the insurance company is trying to cover their exposure to accidents that your children, who are at college live within 100 miles of the house, could cause if they could come home and drive one of the vehicles. This is normal for auto insurance carriers. If your children used to drive you and your wife's cars when they lived at home then most insurance companies would believe the probability of them driving the vehicles when they are in to visit is high and thus they should be covered by your insurance.
If you do not want to include your away at college sons on your policy then you may ask about excluding them. The Pennsylvania Insurance Department notes the first named insured on a policy may exclude any family member from the policy provided that the person being excluded can prove he or she has auto insurance with another company or the Assigned Risk Plan.
Some insurance companies will allow you to exclude a person without proving they have a separate insurance policy, so you would have to see what your insurance company's guidelines are regarding this. If you did this though and your sons at any time did drive the cars there would be no insurance extended to them at all.
There are some insurance companies that may allow you to take your children off of the policy or offer you a discount while they are at school, but usually this is if they are over 100 miles away. So you may want to shop around for insurance rates.
You can contact the PA Insurance Department for more consumer information on this topic and what your options may be.