To clarify, if you transfer Velocity points to KrisFlyer, Velocity would earn an income as they are effectively selling the points. So it's actually "costing" Singapore Airlines.
Depending on the deal between the two partners Velocity/Virgin could be financially gaining by transferring points out.
Ummm, surely that's wrong. To Virgin, the velocity points are like a debt they owe you. You can call on that debt to book flights with them, which costs them money. Transferring points from Velocity to SIA is like transferring the debt off Virgin's books and onto SIA's. So presumably Virgin is paying SIA cash in exchange for them taking the points. While it might be better value for Virgin to transfer points to SIA then to have you redeem them on their own flights, ultimately the best thing for Virgin from a cash flow perspective would be that you don't redeem them for anything.