All of the recent consensus statements recommend more conservative management of concussions in athletes under the age of 18.
In the face of such evidence, the only reason I can think of for allowing a youth athlete to return to play in the same game after concussion symptoms clear is because it increases the team's chances of winning. As I write about at length in my book and on the MomsTeam website, our youth sports culture has become so obsessed with winning that not only has fun taken a back seat, but, more dangerously, safety as well.
Winning, at any level simply isn't worth it, at least when it comes to concussions, which are simply in a league of their own apart from other types of injuries. To the extent our culture teaches that winning is more important for athletes who don't get paid to have their clocks cleaned for a living, needs to change.
If parents know about the rule in advance; if the reason for the rule is explained before the season begins, I think that, by and large, they will see it as putting their child's safety first, which is exactly as it should be in youth sports.
I submit that the alleged lack of scientific studies and the amount of clinical judgment involved in concussion management, and the lack of a consensus, either about grading the severity of concussions or in return to play guidelines, while it complicates our efforts to educate parents on concussions, should not be used as an excuse to do nothing.