Why does a contractor need the approval of his surety to release all?
This is to protect whoever made out the surety. The details will differ depending on the applicable law, but basically, the surety is promising to step in should the contractor fail to deliver. Depending on the type of surety, the comfort supplied to the recipient of the surety can range from a promise it won't go bad' over making good' to payment of money. Depending on the exact type of surety and the law, the surety-giver may step into the position of the contractor, either legally or commercially, or into a similar position. It very much depends on the type of surety and the jurisdiction, there is no globally true simple answer here. That typically can include any claims for payment the contractor may hold or have held against the relevant employer falling to' the surety giver, again, it depends, and the legal mechanics vary hugely. The commercial bottom line, however, is that those liens might also secure claims for the benefit of the surety giver, if the surety is called down. So the surety giver has an understandable intrest in the contractor not simply waiving any liens, because it might need those liens itself further down the road. TL;dr: because the surety giver might also benefit from those liens in the future.