Mac App Store has a feature that lets users to save password for downloading free apps. When you enable this option and installing any free app will be installed automatically without asking you to enter the password.
Incase if you wish you disable this feature then you can do the following
Step 1: Launch System Preferences on your Mac
Step 2: Select App Store on the Settings page.
Step 3: Navigate to Password Settings and select “Require Password” for the Free Downloads option.
This should make the app store to show require password for both free and paid apps.