As @F1Krazy said, it isn't really an inconsistence. A beginner learner - in any area - is first taught the general rule and trained on a certain logic. Later, as they master those concepts and logic, exceptions and finer details are added in.
I'll give you an example from learning English as a foreign language: first, students are taught that Present Continuous is used only for temporary actions happening now and use it in exercises for that effect. Then, the distinction between 'temporary action now' vs. 'facts, routines and opinions' is added in and practiced until both structure and ability to distinguish between the two from context are flowing well. Later, they are taught that it can be used for talking about future arrangements. Even later, they are taught that it can be used for exageration (you're always making noise).
If all that information were given at one time, it would be too much. Moreover, the use for exageration would clash against the most usual use of 'always' for routines and students would get lost and frustrated. Especially children.
Similarly, it's easier to write a well organised text if you're using short sentences and the paragraphs are draconically rigid. That way, students learn - through practice - to organise their ideas in a clear-cut way. As their ability improves, they won't need such rigidity as they'll know when to use the rules, how to bend them and when to break them as a way to transmit their ideas more accurately.