Why can't I make a comment for some projects? Answered

I had a look through the FAQs, but couldn't find the answer, so thought I'd ask here...

There is a project for making conductive paint.  It's a pretty neat ideal which I plan to try.  I wanted to make a comment about the construction method, but when I clicked in the comment box, I got a pop-up telling me to sign-in.  At the top-right corner of screen where it said "You" had changed to "Sign in".

At first I figured it was some rule, for example, either I'm an Instructables Virgin who hasn't posted a project yet so can't make a comment or maybe you need to have a Pro account to make comments.

However, I looking at a couple more projects, I clicked in the comments box but did not get the Log in pop-up.

Are there any rules/restrictions for making comments to projects?