Here's my question: Who is attending these seminars?
Is it members of the same organization that these people have left? If the answer is yes, then I do think there is a level of extreme dishonesty going on.
Otherwise I am very confused. Where I am from it is pretty uncommon for someone affiliated with one organization to attend a seminar hosted by someone affiliated with another (or no) organization.
It's an open seminar, anyone can attend. If members of the old organisation attend (if they're allowed) then that is not the issue of the organising teachers. One more time, they have not advertised themselves as having any present affiliation to the group they left.
If I was part of an organisation that restricted who I can play with or who's seminars I could attend I would be leaving them in a heart beat.