Register/Login/Sign in pages with or without main menu?

I see a recurring pattern in Register/Login/Sign in/Recover Pass/Etc pages of bigger sites, where they avoid using the full length of the main menu top bar. Frequently, they just leave the essentials like a link back to the home page and maybe a currency/language selector.

Is there any good reason for this?

Is it a good or bad practice to hide the main menu (or custom build a smaller menu for the login area)?