I think apart from the business angle the biggest factor is the insecurity of some people. When you are a part of the club people feel a shared connection but when you leave they take it as a rejection of them on a personal level as well since they are still into the club/style/whatever. I think they see it as you basically telling them they suck, and you think you're off to some magical land where everything is better etc.
I gave a head's up to the owner of my old gym and I left on good terms and he didn't throw a big fit about it or anything, but I've heard plenty of stories regarding people leaving that ended badly, with people on both sides of the issue acting like dicks.
I thin that because of my age I am the same age as many coaches and owners...the two schools I have been a part of. Many times they talk to me about the business aspects. In the case of my first trainer he let me in on his financial problems and what not...so although I really wasn't getting what I wanted for the huge amount of money I was paying him I could not bring myself to quit..so I punked out and made up an excuse about not having the money. I got a mountain of **** for that too.
The second place just didn't seem right for me...too many young adults and no true beginner program so I really was getting left behind...and felt no since of comradre with the kids and I was the only one in the class out of shape. Not to mention classes started at 5pm...great for high school kids...sucks for a guy with a 9-5 who commutes 120 miles round trip. So obviously, I left there as well and received a call from my coach...he really wanted me to stay...didn't seem to be financially motivated, but still felt guilty. I gave him some weak excuse as well. I feel guilty that I will probably go to this gracie spot as they have a lot of 35+ guys and a real beginners curriculum...but he will eventually find out I'm there and I'll have to explain it. Shouldn't feel guilty...I'm a consumer...I just do.
I'm very anti-conflict and telling someone that they are not quite up to snuff or the environment just isn't right doesn't work well for me.
This can be a complex situation. A lot of it is I think motivated by finances and ego. There is however also other factors that can maybe cause an instructor to take offense to a student leaving. You have the investment of time put into a student also. For example using MMA lets say you own a small school and you've got a particular student who shows a lot of promise as a fighter. You start working with him when he is about 15 or 16 and train him till he's 18 or 19 and then he drops you for a larger better known school and fights for them. I'd be pretty hurt as a teacher in that instance.
In a more TMA setting its mostly about ego and money I think. My own experience was pretty ugly. I had made an agreement with my instructor to run one of his schools in exchange for private training. The school never became a major money maker but it paid for itself and made a little extra on top of that. I ran it for a year and had all of 2 private training sessions with my instructor. I was getting very fed up but still felt a certain amount of loyalty to him and the school, not to mention my own students. I by chance met another Limalama instructor near by that was willing to continue my training. So I informed my instructor that I was going to be taking some extra training with another teacher. He seemed tense about it but consented.
A month after this started I got a phone call from some limalama higher ups raging at me. I was told I would train with my original teach and only him or be stripped of my rank and thrown out of the organization. There was also some rumors that got back to me of more unpleasant things happening if I didn't fall in line. I told them they can take my belt but not my knowledge and I'll teach wearing a white belt before I'd cow tow to their bullying. I contacted my original instructor who was not at all helpful or sympathetic so then I went to my new teacher who sided with me and succeeded in protecting me from any fallout over this (he threatened to leave the organization with me if they didn't back off and he was far to close to the family and the Grandmaster for them to let that happen). In the end I kept my ranking and left my original instructor but man what a nightmare.
Wow, that's some bullshit.