Sitecore Publishable flag makes it impossible to GetItem() from Master database

I'm using sitecore 6.6.0 (rev. 120918). From the sitecore admin portal, I go and turn off the Publishable flag of an item (see image).

After this, Sitecore.Data.Database.GetDatabase("master").GetItem("{itemID}") returns null.

If I turn ON the Publishable flag again, GetItem() returns the correct item. What's the reason for this behaviour? Publishable setting controls the ability to publish to the Web database. Why does it effect the GetItem() API call to the master database?