In Harry Potter and the Half Blood Prince - Chapter 9, Slughorn says (emphasis mine):
"I've prepared a few potions for you to have a look at, just out of interest, you know. These are the kind of thing you ought to be able to make after completing your N.E.W.T.s."
One of the potions in question is Amortentia - the most powerful love potion in the world. Why exactly would anyone in the Wizarding World ought to have to be able to make it?
It is no doubt a complicated (i.e. advanced) potion to make. That is why it would be required for a N.E.W.T. exam. The difficulty in making it demonstrates a level competency in potion making.
As to the ethics of teaching students something like Amortentia, almost everything about the magic world can be used maliciously if they want to. They spend a lot of time transfiguring objects, and it has been shown you can transfigure humans as well. So a gifted student could do that to Muggles (or whomever) and it would be just as unethical as doping someone with a Love Potion.
Slughorn had a specific motive - he wants to find those naturally talented students so he could form more connections with those wizards and witches most likely to become powerful in the future. To do so he not only needed to present difficult tasks, but also suitable motivation. The boring but complex potions will hardly generate the interest that a love potion would generate, and it might be enough to bring the cream of the crop to the top.
Even if that weren't the case, education often teaches hard concepts one may never use in order to demonstrate principles, and strengthen the foundation of one's understanding of a topic. In the same way that understanding a complex and difficult recipe can help a chef prepare simpler recipes when called on, understanding a difficult and complex potion will make understanding easier potions - and creating them from scratch - possible. You may never use calculus as an adult, but you know that a pool with a sloping bottom will fill more slowly as it nears the top despite the same rate of water input, and you'll know why.
Further, magical practitioners also need some experience with the more nefarious potions so they can identify them when needed, or avoid them if someone is attacking them in a hidden manner. This could be taught in Defense Against the Dark Arts, but it properly belongs in potions and teaches several things there.
I suspect this was also simply Slughorn's teaching method. While Snape focused on the foundational ingredients and preparation of potions, Slughorn discussed the high level concepts and results one could obtain.
Taking your quote here:
"I've prepared a few potions for you to have a look at, just out of interest, you know. These are the kind of thing you ought to be able to make after completing your N.E.W.T.s."
Note that he says that they are the "kind of" things, not the "things." This indicates to me that once you have passed the NEWT and are acknowledged as having magical mastery at that level, you should have the ability to follow the necessary directions and successfully perform the necessary and intricate steps to successfully make the potion.
I used to teach organic chemistry. I don't expect anybody to remember years later how to make aspirin. But they should have learned well enough to remember how to do a filtration step, or how to do distillations, or any number of other chemical procedures.
Chefs similarly were classically tested on the quality of their hollandaise. They may never need to make it, but they would be expected to have the skill necessary to be able to do so.
I know plenty of chemistry students who have learned how to make some pretty serious illegal drugs in university labs. Obviously this is just for displaying the technique which is what I would imagine this was. The students are never actually meant to do this again but it sounds cool and gets their interest.
Certainly these things are the only things I have heard them excited to tell lay people about so it seems to work.
