Why are some topics locked from replies?

which topics are locked?

(are you sure they are?
I had the problem once, that I could not post. but just retrying later solved the problem)

the 'reply' button shows 'locked' so no way to post

There are only a few locked topics.

For example:
http://www.scientistsolutions.com/index.php?a=topic&t=1653

Which was locked for a good reason, there are others like that.

I've reviewed the other locked topics, and unlocked some in bioinformatics -> text and data mining, which I couldn't see any reason for locking.