Field Permission Issue even without Field Permission Module
Sorry to trouble you guys, but I figured it out just now. It was really a silly mistake from my side, but I had been hitting my head ever since. 

Anyways, turned out that I had not provided the permission to the user for that particular text format!! The article was created by admin using "Full HTML" but the user had no permission for that format.

Anyways, I figured it out using a hack method which was really cool. 

The text box was showing this message when it denied permission : 

This field has been disabled because you do not have sufficient permissions to edit it.


I copied this and searched for this complete sentence in the contributed as well as core modules' directory. The "culprit" was filter.module. The function which was doing this was properly commented and said that "deny the users with no permission to the given text format".