I'm a very happy user of both OpenBSD and GNU/Linux systems, but what
I don't get is,  how is limiting a users choice in what he/she runs on
his/her system more free than one that doesn't?

Absolute freedom is to be able to do whatever the hell you want to
with no limitations placed on you whatsoever.  By this definition,
public domain is the only truly free "license".

I understand and appreciate the "freedom" that is defined by both the
BSD and GPL licenses; that of ensuring the authors continual right of
ownership.  However, in terms of true freedom, both have limitations in
place.

Not that I disagree with the limitations they have, in fact I support
them both as the current systems in place require the need to protect
your original copyright.  It's Utopian for me to think this, but in an
ideal setting, there would be no need for any licesnes and everything
would be available in the public domain.  But since we are arguing about
which license ensures more freedom, I think they both fall short of
what it actually means to be free.