There are many, many things like that in the game.
"I want to attack the baddie with my sword and then punch him."
"Are you a monk?"
"No."
"Then, sorry, to make an off-hand attack you must be using two light weapons."
Did we just tell the player that they "aren't allowed to make that decision"?
And on and on.
I mean, I support the sentiment of your post, but it goes without saying that in this game we all love to play (and waste away work hours discussing) there are constraints that we follow. The Frightened condition is one of these mechanical restraints.