Would you be equally comfortable with the DM telling you that the orc threatening your character fills your heart with primal fear? Note, being afraid is one thing. The Frightened condition, with it's game effects, is not simply being afraid. It is a mind controlling effect that forces the player to perform (or prevents him from taking) certain actions. This goes beyond simply, "You're really scared", which, again, is something a lot of players would have a serious issue with, and enters into the realm of the DM taking over player characters.
Magical effects are accepted as a means of doing this since it is literally mind control. Fair enough. No one bitches when their character does something because of a magical effect. But, an orc threatens your character and you are forced to move away at top speed and can take no other actions simply because the DM rolled a good Intimidate check (with a DC that was also set by the DM, since Intimidate is not an opposed check). Or, your character must be friendly to that same orc simply because I "succeeded" on a persuasion check?
Yeah, I'm thinking a lot of players are not going to pat the DM on the back for that ruling.