5 year olds in booster seats

My son started preschool and noticed a lot of parents putting their 4 -5 years olds in regular booster seats. I don’t know if it’s illegal but for sure I know none of those kids have the weight and height requirement. I caved a got booster seat for my son for my husband car. It’s a 70s model and the seat belts in the back don’t extend enough for a toddler car seat. After noticing that it’s just so much easier for him to get in and out (he can buckle) himself, I’m thinking about getting one for my car already. I don’t know what to do. Am I overreacting?