Why is this a "security" issue? People know that other people can see their "last seen" date, but usually they don't care so they left it. Hell, I leave it public as well. I couldn't care less about this tiny details.
Now, if you tell me that by leaving public my "last seen" date, people can get more data about me (e.g., with whom I talk to), well then yes, I would call that a security issue.
Now, if you tell me that by leaving public my "last seen" date, people can get more data about me (e.g., with whom I talk to), well then yes, I would call that a security issue.