Do you learn languages for the sake of learning languages, or for another purpose? If you learn because you enjoy simply enjoy learning another language, or love the beauty of that language, then you are 'learning for learning's sake'. If you are learning because you have to speak another language for work or for travel, then you are learning for another purpose. Most people need a purpose for learning a language, and do not learn just for the sake of learning. But once they start and they enjoy it, then they might say: I learn English for work, and also for learning's sake.' In this case 'We do not want prohibition for prohibition's sake (because eg. drinking alcohol is morally bad)' we need to consider the other important factors so that we can make a good decision.

Why do you prohibit something? (or make something "not allowed"?) There can be different reasons, right? But in some cases people prohibit things just to prohibit something - anything at all, no matter what it is exactly. Just to be able to say "I say this thing is not allowed and it means it's not allowed. End of story." - to see if everyone else will obey, no matter how crazy the idea is. When it's done not for any good reason, but because people like that they have the power to prohibit something to others. That's "prohibition for prohibition's sake". Or the same structure can apply "anything for its own sake". Like "driving for driving's sake" (you can also see this written as "driving for driving sake" sometimes)- not because you have to go somewhere, not because you need to be driving, but just because you like the process of driving.

I don't understand "-we do not want to not allow something for the sake of not allowing it" Could you explain it to me in detail?

Anyone who talks like this is someone you should not be trusting. LOL Sounds like a politician. Anyway, here's my take. In other words this says... "The discipline (practice of training people to obey rules or behavior)--we do not want to not allow something for the sake of not allowing it; and it must not be a sudden change--it is no use forbidding a thing one day and allowing git the next." So they're talking about a law or something and they say how we shouldnt change it for the sake of changing it, and if we do change it, it shouldnt be an erratic decision.