That one's common across a lot of styles. Personally -- I see it as courtesy. I won't necessarily throw a student out for cross training -- but I'd like them to let me know. Part of that's so that I know what they're up to, or potential injuries, part of it's to steer them away from ripoffs, and part of it's about whether I'm wasting my time with them. If they're not practicing, but are chasing something else a lot harder -- that's where they ought to be.I'll add one: You can't train at another school (same art or different art) without asking or mentioning it to the first school.
This kind of bugs me... If Master A simply collects the money to cover the KKW fees and wire costs or the like, but you've been training under Master B who charges an extra $200 for no reason but to line his pocket -- why protect him. (Note that this is different from "master shopping" to find someone who'll file or recommend or the like because your own master isn't doing so for you.)And you can't get KKW registered from another Master who may charge less.