I legitimately do not understand what is either shocking or oh so infuriating about Devil Fruits being Haki restricted. They had to be. If Law via his devil fruit was capable of one-shotting Big Mom why the hell would he have ever worried about somebody like Doflamingo? If some OP devil fruits worked without restrictions and upper limits, it is a complete I win button against all other characters.
If Law was capable of using his powers on people like Doflmaingo without restrictions, nothing about his team up with Luffy would make sense. Haki was intended as an equalizer to devil fruits, Garp and seemingly Rodger were top tiers without fruits because of their Haki.
If devil fruits were able to work on a person no matter how strong their Haki was, that would never have been possible since people with bullshit fruits like Law or Sugar would have one-shot them somewhere along their journey.