With respect specifically to Gardnerianism, it includes certain specific sorts of things which are considered oathbound. This includes the following three categories:
Part of training is a weeding process. For the student, it is the time to weed out the teachers who are power-happy, want to be 'gurus' or are simply jerks. For the teacher, it's the time to weed out the student who is simply not suited for an oathbound path. After all, there are a number of folks who just "don't get it". Here is a sampling of the weird logic they attempt to use.
Picture a child, say a pre-teen adolescent, saying "but everyone else gets to do it" in an annoying, whiney voice, and you will grasp the maturity level of someone who uses this sort of excuse. You'll probably also get a good idea of how much patience the people who hear it have with it. Of course the obvious comeback is to ask them what anyone else's actions would have to do with their oath? Do I have no obligation to keep any promise I make, because someone, somewhere, broke theirs?
#2 Well telling someone the 'secrets' doesn't give away the 'experience'.
Actually, this is a very good point. With respect to a very old oathbound tradition that took place in Elusis, there was a very clear differentiation between these two very different concepts.
The details of that religious practice, the names they used to address the divine, the rituals themselves - all this was collectively called the Lesser Mysteries. The actual experience, the connection with the Divine was called the Greater Mysteries.
The Greater Mysteries cannot be kept secret - for there are many ways to connect with the divine. Makes sense; if you cannot communicate it, then you can't promise to keep it a secret. However the Lesser Mysteries were treated a bit more seriously. Obviously, since this is the part you can reveal, since this is a promise that can be broken, this is the promise that matters. Betrayal of these was punishable by death.
Or, as a number of long time members of the Craft have been wont to point out: "in the old days, we just killed them." Looking carefully at the sanity, reputation and personal lives of those who actually have broken their oaths, well, maybe in the old days we were more merciful.