What I don’t agree with is your view of how someone should learn a humanities subject. There are facts and evidence - eg in history, dates of events, primary evidence, secondary sources etc, so plenty you have to know about and prove you know about, but how you piece together the facts and evidence and draw conclusions from it all can differ, depending on the importance you place upon certain things, or the particular angle you have chosen to focus on. Likewise with law - yes, you can rely on someone else telling you “what the law is,” but you’ll fail as a barrister if you always accept someone else’s interpretation, and nobody will want you to represent them if you don’t understand how to argue their case and instead roll over and agree with everything the other side says and how they interpret precedent. Your whole purpose is to distinguish your client’s case from a previous one, or to point out the similarities with another case and its judgement and why that should apply here - there are always nuances, or points of difference, none of which you will appreciate if you expect someone else to do all the work for you so that you can make notes and ask what will be coming up in the exam and what you can ignore.
If you don’t read widely for yourself, you’re just parroting what someone else tells you to think without ever bothering to look at and interpret the evidence for yourself, which doesn’t take you any further than A-level. Of course, therefore, you must read widely, digest what you read, and then back up your own conclusions with evidence and facts put together in a way that suits your own argument, whilst taking into account other viewpoints and why you agree or disagree with them, having actually bothered to read them. If you just slavishly copy what the lecturer tells you to think, you haven’t actually learnt to think for yourself, you are pretending there is only one way of viewing something that is actually multifaceted. And you are welcome to read textbooks - they signpost you to sources that might forward your own argument, summarise things you have less time to go into in depth, give you points you actively disagree with, etc, and your own tutor will provide you with reading lists to help guide you or get you started, or from which to select your sources.