Is it just me, or does it disturb anyone else that an Instructor banned by one agency can so easily move to another?
It depends on why.
Agencies have different policies. For example, PADI, SSI (GUE?) and some others do not allow an instructor to arbitrarily add material to the course and then test students (and possibly fail them) based on that material. PADI's philosophy is that THEY decide what needs to be taught, not the instructor.
Other agencies (NAUI, SEI, BSAC, CMAS for example) give their instructors a VERY long leash and don't police what is being taught very well, if at all.
PADI's approach has the advantage that it is agency based and very consistent over instructors and geographical regions. It makes their modular system and system of referrals possible. The down side is that some instructors feel (in my opinion erroneously) that they are unable to adequately address local conditions.
The approach of some other agencies is not agency based but instructor based. It has the advantage that outstanding instructors are not hindered by the agency in delivering their best possible course. The disadvantage is that (a) some instructors think they are FAR better than they are, and because delivery is controlled by instructors, they are not limited in the depth of crap they can deliver. and (b) there is no "lowest common denominator" so if you have been trained by instructor X you never know for sure if you're ready to take a course from instructor Y.
So all of this is to say that suppose you were kicked out of PADI because you kept adding skill X to the course. They would try to re-calibrate and re-train you for a while but it's possible that the differences in opinion are unbridgeable and you decided as an instructor that you couldn't live with this limitation. (it is unlikely that PADI would actually kick you out for a difference of opinion. The instructor would usually take the initiative). It is, therefore possible that you could then cross over to NAUI and have a free hand in teaching skill X..... it could be a better match.
There are many examples of such scenarios. All of this is very theoretical.
In most cases an instructor who has been banned (regardless of agency) has either done something very serious, very unethical, very unprofessional or illegal. In other words, regardless of agency, the agency must have felt that they had no other choice for either their own legal protection or for the protection of students.
R..