Okay so nobody in Kashima Shinryu trains Aikido, yet they understand Aikido so well
that they know they don't want new students who practice Aikido. But that should come as no surprise since the ban against aikidoka is built into the okuden of the ryu - so they've obviously banned aikidoka from training for hundreds of years before Aikido ever existed!
Sorry, this just gets sillier and sillier to me. It makes more and more sense, though, particularly if Tissier's teachings are as widespread as you say, that at some point the ryu decided that the Aikido organizations posed a real threat to the proper continuation of their art. They don't want to be invited to Aikido seminars, they don't want people coming to train and then showing their friends at the Aikido dojo stuff they don't understand yet, etc. Presumably other arts don't pose as much of a threat in that regard because they are not open-ended like Aikido is.
Sorry for the argumentativeness, Carsten - it's not you I am arguing with, its this omote cultishness that seems to impy that Aikido training damages someone's budo permanently.
I think it is fair to say that there are several elements of the problem, from the KSR side. From my conversations with one senior practitioner of KSR, (and this is a summary of my understanding, not a verbatim quote) the first part of the problem is the variance between the comparatively homogenous and settled inner kabbala of KSR on the one side, and the comparatively heterogenous and unsettled inner kabbala of aikido on the other. The second part of the problem is the tendency of many aikido practitioners to study a koryu art for the primary purpose of improving their aikido, as distinct from the purpose of learning and inculcating the complete movement system and attitudinal structure of the koryu art. The third part of the problem, even if the first two problems are rendered moot, is that yes, whether you like it or not, it is precisely the view of senior KSR practitioners that Aikido training most certainly can permanently damage one's ability to develop and manifest the teachings of KSR. The resistance of aikido practitioners to this possibility is, from their perspective, evidence of the fundamental problem.
For a pernicious thought experiment, try imagining what might happen if Kenny G were to ask Branford Marsalis for saxophone lessons so he could get a little of that New Orleans feeling into his next release.