Let me turn that around. What makes this particular form of exploitation so special that it warrants a rule? And if it's so special, why isn't the rule stronger?
This may be wrong, however it was what I understood at the time the rule was created.
The rule against exploits or playing counter to game mechanics in general is not explicit, but covered under "Play fairly, as with friends". This in itself allows for wide ranging interpretations. Many people would argue that, if we think in the frame of mind of our nobles things like strategic secession is logical, since nobles being aware of the recruitment limitation would seek to overcome it. If I recall correctly Tom's opinion was that nobles would NOT see their realm give up land and lose prestige for the sake of efficiency, that appearances were far more important. Thus a rule was born to cover the specific issue.
Why is it not stronger? Most of the rules in BM aren't. Part of this is no doubt tied up in Tom's love of social experiments. I believe another part is to prevent overly restrictive rules from being used to prevent fun and interesting play. This is tied up in the hybrid nature of the game. If it was only a strategy game without the strong RP element I believe things like strategic secession would indeed be outright banned. We want this game to be MORE then just a strategy game though, and so the rules must be flexible.
Finally, this is part of the "policies" These are things that have grown over time from discussions with Tom and messages covering specific events. Apart from Ethan trying to collect them and place them on the wiki, I don't believe anyone has tried to sit down and codify them as has been done for things like the Social Contract. They are like a informal series of precedents, and I think the ambiguity they present is in part because of their origins.
