In Oklahoma they only have to be age 5 or less to be in any kind of child care seat. Once they turn 6 they don't have to be in any kind of seat if the seat belt sits in the right place across their chest.
This is what I found when I looked up your states laws.
Ages 4-8
"Children age four (4) through age eight (8), and measuring less than four feet nine inches (4'9") in height, must be secured in a belt-positioning booster seat system, meeting federal motor vehicle safety standards in the rear seat, if available, or according to the child safety restraint system or vehicle manufacturer's instructions. (Note: If the child is not between age four (4) and age eight (8), but is less than four feet nine inches (4'9") in height, he/she must still use a seat belt system meeting federal motor vehicle safety standards.)"
http://www.tn.gov/safety/newCRD.shtml
Belt positioning is the key phrase in that whole section. If their seat belt fits across their chest they don't have to sit in anything. If they did it would make the seat belt not fit right.
If those parents want to take the chance on getting a ticket if they are stopped then let them. If they don't get stopped then they are fine IF the seat belt is fitting where it's supposed to be.
My ex SIL got stopped several times with my oldest grandson in the vehicle with him. Not in a car seat. They asked his age and since the belt went across the right part my ex SIL told them that if he was in a booster the seat belt went across his stomach. They said he had made the right choice and he NEVER got a ticket, even when my grandson is 4 or 5 years old.
So the seat belt position is the whole deal. Kids get hurt due to the seat belt being across their throat or neck. If it is across their chest they are fine.