NodeJS unsafe-perm not working on package.json

I'm trying to run a npm install command with a preinstall script at my package.json. I know it's antipattern but I need to run some scripts as root.

It's working fine by adding a .npmrc file containing unsafe-perm = true to my root directory. But it's not working by add a config property in my package.json file:

   {
     "name": "foo",
     "version": "1.4.4",
     "config": {
        "unsafe-perm":true
     },
     "scripts" :  { 
        "preinstall" : "npm install -g bower"
     }
   }
   // It is not working

According with NPM config docs it's ok adding this property in my package file. I want to understand why it's not working.