Also, I worry that on these unclear cases, different mods are moderating different boards, and we'll end up in a situation where mod A thinks it should be on X, and mod B thinks it should be on Y. With the current system, the user will be super frustrated and the mods won't even know it's happening. Much better to have a paper trail of moderation actions on a thread than to have the user remake the thread anew because of this.
In the current implementation, I agree. But, as Daen said, in the long term, riot intends to let us make our own boards, with our own guidelines (although everything will, of course, follow the universal rules). If something violates those guidelines, we'll report it to mods, who will then read the guidelines, decide if we're right about the violation, and take the appropriate action.
Since player-made boards aren't ready yet, Riot is forced to try to come up with boards that fit our communities themselves, and that's where these issues arise: because we aren't defining the communities / board guidelines ourselves, we have to try to understand what Riot intended each board to be for, which isn't necessarily easy.
An example of how it will work in the future is the Role-Playing board. You'll notice it's not listed on the right, yet (the link is accessible through the old RPing forums). That's because they're in the process of defining their own guidelines.