E.g. a guy watching torrented Fifty Shades of Gray to understand what are the people on the internet fussed/excited about.
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You might make arguments why pirating content instead of paying is always or often ethical (I would likely disagree), but the reason why you are watching the movie doesn't make a difference (with few exceptions; if it turned out that somewhere in the background shot out of a window we can see a car being stolen, then it would be ethical for a police officer to watch that scene without paying).
That would depend on your ethical principles. Some people have no objections to theft of physical property; there are those who have no truck with the concept of intellectual property (so it would be wrong to take an artist's physical painting, but okay to make a copy). I assume you generally accept the general moral principle that a person owns the intellectual "content" that he creates and this has the same property rights as he would have to his vegetables and house.
Moral philosophy also admits "exceptions", for example killing a person is immoral, but there are contexts where it is not -- for instance, if the person is attempting to kill you, then you may kill him if it is necessary to preserve your life. Such an "exception" follow, at least in an egoist framework, from the purpose of a moral code which is to prescribe how to live your life – being killed obviously contradicts that purpose.
So: what could possibly justify an exception for the purpose of understanding the fuss? Well, it would be something like, "The wish to understand a popular internet trend justifies ignoring the property rights of others". But this is obviously not a general principle, it's a post-hoc rationalization. I can't conceive of any reasonable principle that would lead to such an exception.