However, I want to focus on a particular paragraph of his:
GPL fans said the great problem we would face is that companies wouldtake our BSD code, modify it, and not give back. Nope -- the great problem we face is that people would wrap the GPL around our code, and lock us out in the same way that these supposed companies would lock us out. Just like the Linux community, we have many companies giving us code back, all the time. But once the code is GPL'd, we cannot get it back.
Social issues aside, this section implies that it is the users of the GPL that are at fault for locking the BSD users out of re-using the dual-licensed code. However, the reality is that is the combination of the licenses that prevents it. The users of both licenses choose their licenses for a reason, and must deal with the ramifications, not the least of which is that BSD code cannot borrow from GPL code without changing the manner in which the BSD code may be used. So, when Theo chose the BSD he made a conscious decision to restrict himself from re-using GPL code, so ... it's not strictly the fault of the GPL users. The OpenBSD guys could choose and use a license which doesn't have this restriction if they wanted to.
In other words. The cause of the restriction is based on your frame of reference or point of view.
 - http://undeadly.org/cgi?action=article&sid=2007090104165