In the old days, if someone didn't want to tell where they learned something from (because they didn't have permission, or they had a falling out with their teacher, or they invented it), maybe they'd say "a wandering monk showed me", or "I visited a hermit on a mountain."
We can't really use those excuses anymore. The people who try it end up looking like complete scam artists.
Things change, grow, evolve. Some people embrace that and encourage it, and others try to stop it and keep things static. There doesn't need to be any disrespect involved...but sometimes it's unavoidable, when your teacher is of the latter school of thought and you are of the former. Your teacher demands that you only teach other people what he taught you, unaltered in any way. You have grown as an artist (as we all do through years of practice), and have new ideas and do things differently, in addition we learn from various sources whether we want to or not. Your choices are: wait until your teacher passes on before doing what you want, or "disrespect" him and go in your own direction (without permission and maybe not being allowed to even mention officially that you were his student).
Another factor today is business and legal stuff like copywrites. In the case of some chains of schools, you can't legally call your school by the same name as them without permission, being a part of the organization. You might not even legally be allowed to claim them as your teachers unless you are in the group and following all their rules. So do you give up your martial arts pracitce? Acquiesce and just follow all their rules, despite your misgivings? Or teach what you want and how you want, just call it something else and accept a level of "shadiness" in the department of lineage?
Though I prefer honesty in all situations, I don't hold it against anyone who teaches without the permission of their teacher, or makes changes to curriculum. The martial arts skill will speak for itself. Names on certificates and historical ties don't automatically confer skill or "worthiness". I won't judge anyone without practicing with them or at least seeing how they practice. That's the only way to know how sincere they are, and how skilled they are.
"I am a servant of the Secret Fire, wielder of the flame of Anor. The dark fire will not avail you, flame of Udun! Go back to the shadow, you cannot pass!"