I personally think people may teach as they will. It is simply none of my business who teaches who, and no one should be forced to teach a person or group against their will. That said, it will significantly benefit the practitioner of Aikido to come in contact with a wide variety of other humans during training. Limiting a student's ability to contact a wide variety of other practitioners is at some point going to be doing the student a disservice.
Of course, people are free to do what they like - that doesn't mean, however, that a particular course of action is wise or advisable.