Rather than thinking of rolling back an rolling back an editedit (i.e., from a given revision), the system thinks of rolling back to a previous version/revision.
For example, if you think that Revision 2 was a bad edit, then you should rollback to Revision 1 by going to that revision in the edit history, and clicking Rollback on Revision 1.
Here's how it looks on your question here:
Since Revision 2 is the current revision, you can’t “rollback” to it — only to previous ones.
