The rules do not make a lick of sense rp wise, but that's not the point, the point to to make Edo less op and make it so less people have it. It doesn't make sense that someone forgets learning a jutsu canon wise, but canon wise, an Edo use is also allowed to summon as many zombies as he wishes, such is not the case in SL.
The Student teacher rule came as a result of a 29 page discussion on Edo Tensei (Which many of the people in this thread were apart of) It should not be banned, as Edo Tensei has been limited and represents one of the few times this community got together and got something done, which is grand.
The rule first applied applied to Boccheire and myself. While Bocc was alive, I couldn't teach, and while I was alive, he couldn't teach. That is the essence of the student-teacher rule. Though it got complicated when people tried to steal the technique without killing the user. Thus amendments were made, notably if you steal the info, you replace the user you stole it from, and they lose it. This rule was made to limit the amount of users and has been very successful.
If you count Shinro as keeping it after being inactive, there can only be six possible users, if you cut him out, four. This is how it's been, and how it's worked.
So right now, as follows
Warren may only teach Edo Tensei if Keito dies
Keito may only teach if Warren dies
Kamui may teach is Masane dies
Masane may teach is Kamui dies.
This is if nobody get ET pirated.