Throughout One Piece, a recurring bane of devil fruits is Seastone. While it does completely seal off their devil fruit powers, why can't they just use Haki to break it apart? Haki doesn't require anything other than willpower to be used, and is extremely strength-imbuing, which should make it pretty easy to break the cuffs, given that Luffy seems to have broken through objects of much higher density and mass with Haki.
As mentioned in the wiki:
Upon contact the material drains the victim's body of energy, and stops them from controlling their Devil Fruit powers - having the same effect as throwing them into the sea.
It doesn't simply lock the DF powers, but drains the energy of the user. This is why DF users are unable to even swim in the sea. Otherwise, if it was only blocking their DF power, they could have still stayed afloat if they had the physical stamina.
Unlike the sea which inhibits DF users to the maximum, seastone itself can have varying degrees of power
The degree of "weakness" induced by the Seastone depends on its density and composition; for example, Seastone handcuffs rob the prisoner of their abilities, but still allow the prisoner to move their bodies around normally.
So it just becomes a matter of ensuring that the prisoners don't have enough power in them to use Haki properly. As mentioned in Chapter 935 (Wano arc),
Luffy has indicated that being weakened by Seastone affects his Haki, as he was unable to use it with Seastone handcuffs on and became able to once he had the cuffs removed
My personal assumption had been that since the DF user's body is drained of power in the seastone cuffs, the result after a Haki boost was still not enough. Busoshoku Haki in general only acts as invisible armor, which can then be used in an offensive manner due to the users own attacks or weapons. It only makes things harder/denser and doesn't grant more 'force' or power to them.
However, considering the above excerpt which seems to indicate they can't use it at all, there might be some other yet to be introduced mechanic.