Well through my experiences with being taught music. I have had music lessons in both a classroom setting (15-30 people) and a one-on-one setting with just me and my teacher.

In both cases my teachers always made sure to teach me both theory and the practical application of my instrument. So I would have to say that they enjoy the process and the product. Besides how are you susposed to get a good student if you don't try to teach them?

And I'm also aware that there probably are lots of bad music teachers, but theres lots of great ones aswell.