If the town chooses to humanely euthanize, the HSUS won't stand against it as its a humane method and a choice of the town. There's no legal reason for them to step in. Spaying and neutering feral cats doesn't solve the problem of existing feral cats. Not only that, but they may not have the funding for spaying and neutering all of the cats. So long as the cats are caught humanely and treated so until the end then they aren't doing anything wrong.