I think there are three different situations we should think about.
First one is when some count rebel against your vassal duke. King should be allowed - but not obligated - to help his vassal. It's totally frustrating that I am supposed to kill some peasants rebels, but when some guy want to get the title I personally took from him for his questionable loyalty and gave to some good man, then I cannot do anything.
The second case is when some of my vassal has CB on my other vassal. In that case I think king should not be allowed to act, because
"In a christian feudal realm, If a vassal goes to war with another to get one of his titles, it means that he has a valid casus belli. Therefore, he has a lawful right to seize the title."
But there is also third situation. When some *$%&%@ think he can bully my son/daughter/friend/brother-in-law. IMHO they should be under king's protection just because they are his relatives or friends.
Note that in any case, I would prefer if the game allowed you to do what you want as long as you assume taking tyranny opinion penalties corresponding to the infractions you committed, instead of completely forbidding any attempt.
[again, would applies for the feudal governments (except error from me)... for other type of rulership, maybe not]
first situation: I totally agree with you... with no tyranny penalty if you originally revoked the duchy with a lawful reason, before giving it to a more loyal vassal
second situation: I totally agree with you but you totally agreed with me so in the end, I totally agree with myself which is quite comforting for my mental health (maybe)
third situation: In the game as it is now, the *$%&%@ needs a valid claim to bully you son/daughter/friend/brother-in-law. Then he has the lawful right to do so. I think you should have the right to help your son/daughter/friend/brother-in-law but I guess, with a tyranny penalty (reduced if he is your kin???). For nomads, there is the "humiliate" casus belli which can be used to to bully another nomad realm or another clan from the realm (and does target any title) but unfortunately, they are not valid for other governments.
But im playing as the byzantines!!!! I AM the Eastern Roman Empire ! EMPEROR OF ROMANS!!! I am not bound to feudalism like the barbarians of europe.
i granted my Son, and heir a kingdom and some upshot vassal has usurped that kingdom from him.
now in real life this would have caused a civil war within the empire, and vassals would choose who to back.
i think this is a feature that is truly missing from the game, as an emperor or king, i should have the choice to join my son's war, and then an event should fire which would let the vassal decide if he wants to continue fighting and be branded a traitor to the realm, if he says yes then it becomes a civil war within the realm and vassals choose to either support the throne or support the traitor.
I have no idea for the Byzantines... no idea how their empire works, never played as them... I guess Crusader kings was originally built around simulating feudal realms and they based the Byzantine empire partly on the feudal mechanics... that's why some features are strange for some other governments.
Anyways, what you describe is fascinating. And just because it would bring so much depth, to make civil wars begin like that (and maybe in the end, making empires fall), I would love to see that possible in CK2. ^-^
edit: Actually, this situation is similar to PK_AZ's third situation... PK_AZ's third situation should also increase the risk of, and eventually trigger a civil war at the scale of the realm... Would be interesting no? ^-^