You have to remember that Wano is a big supplier of seastone to the World Government.
Take some lab rejects, trade them in from some seastone. Not a bad deal. At the start of Wano, the CP0 were haggling over Vegapunk himself with Orochi which is far more outrageous. Kaido might be only Yonko the WG makes these kind of deals with.
The Numbers seem entirely dysfunctional right now. It could be the booze. They are not leading troops or becoming the front line fighters, these drunks just show up whenever they want and smack their own men. Does the Navy really want officers like that? Even using them as gatekeepers in Impel Down might lead to more trouble than stabilitiy, provided they can even fit through the front door.
The bigger loss is for the Beast Pirates. Kaido wants a crew entirely made up of Devil Fruit users, yet he's selling seastone to his greatest enemy. But he's the man who was convinced by Doflamingo to become the exclusive consumer of SMILE. The Numbers could just be one more iffy deal in a long history for Kaido, but I would like to think it was an underground deal with a corrupt member of the WG. Like the deal Nezumi and Arlong had going, or Doflamingo and his insider Vergo.