I can’t think of any reason why someone should have to wait 6 minutes to post after logging in. I don’t think it is unreasonable to believe that someone may read many posts while not logged in, find something interesting, log in, and try to post.
It's probably just a bug that's not really worth fixing, just like logging in for the first time tells you something failed, but if you just registered you can ignore the message (I don't remember the exact wording).
At least with the first-time login bug (I know what you are referring to), the user will be notified and is given clear instructions on how to resolve the issue. Having to wait 6 minutes after logging in is not obvious to anyone new to the forum, even if they read the sticky threads in meta.
I am not familiar with the SMF codebase, but I imagine it would not be terribly difficult to not count someone logging in as an action that a user needs to wait an amount of time in order to post.