Most children have tantrums sometimes, and you need to be a lot more careful with those who don't. My 8 year old still tantrums. He yells for a minute, gets sent to his room, and that's it. No sulking, no grudges, we know exactly how he feels. Of course, we're working on more appropriate ways to express emotions, but that's an ongoing process. It takes children years to learn how to identify and process emotions appropriately. Earlier methods of childrearing may not have permitted tantrums, but neither did they permit boys to express emotions appropriately, with the result that many men still cannot.
If you don't understand the rationale behind modern childrearing practices, than politely ask. Otherwise, you don't criticise what they do in their own home, if the children are obviously happy and healthy.