Admin privileges requiring for installing a package build though Packages
The installer is actually doing the right thing by asking for an admin password. Users without admin rights do not have the ability to write to the /Applications directory on a Mac.

Without knowing more about your installer, I suspect the request for an admin password is coming from Apple's Installer.app, which is the application that your installer is working with to install your application. 

Installer.app is detecting that the non-admin account that's running the installation doesn't have admin rights and that /Applications doesn't allow non-admin users to write to it. To resolve the issue, Installer.app is requesting that an admin account authenticate the installation and authorize the needed rights. Otherwise, the installation will just fail.