Insurable Interest vs. Ownership

Would you please help me with a personal auto question. Our insured is the named insured on his personal auto policy.  He has two sons that live with him one 18 and the other 20.  The 20 year old has a car titled in his name and it is listed as a covered auto on our named insureds policy.  I've been told that because title is held in the 20 year olds name, he must have his own insurance.  Why? I see under exclusion B3 there is no liability coverage for any vehicle, other than “your covered auto” which is owned by another family member.  But “ your covered auto” is defined as “any vehicle shown in the declarations.  So, despite ownership status , there is coverage despite the title being in the son's name, correct? Why can't each of the sons own their own car and still have them on their father's policy as long as they are living in the same household?

Maryland Subscriber