An argument that's often made is that piracy harms content creators. That music piracy impacts the revenue that artists earn through royalties, etc. The gist of the argument is "piracy hurts those who make the content you love".
Whether you believe that or not, do you know who this DOESN'T apply to?
Do you know who receive NO royalties for their published work, and in fact must PAY just to get published? Academics.
You should feel no guilt for obtaining free scientific papers. The authors are losing nothing; their research is being seen by more people than it would otherwise.
So why do they pay to get published? Reputable academic journals serve as a clearing-house. Getting your paper published in a journal is a mark of quality, indicating that your paper has been peer-reviewed and accepted as a valid contribution to science and knowledge. And I have no problem with this; people pay for certifications all the time.
What I have a problem with is that the journal, who have already been paid to publish the article, then turn around and charge the general public for access to the article. (I guess it's usually free if you're a student, but still.) Essentially the journal is taking ownership of, and restricting the dissemination of the scientific knowledge that they were paid to publish.
Publish: from the Latin publicare, "to make public". If it's behind a paywall, it's not publically available; so technically, it's not actually published.
Consider also the rise of "predatory journals", something that can only exist because of this business model. Predatory journals basically exist to scam academics into "publishing" their work in a journal that is neither peer-reviewed nor reputable. They also make money off those who want to advance an agenda; predatory journals will happily take money to publish just about any paper, including those which were outright lies or would never pass a real peer review.
The only ones who benefit financially from all of this is the journals themselves.
So, pirating academic papers may harm the journal it's published in, as they lose out on potential access fees. But the authors don't lose anything financially. And they also don't lose any value in publishing; they are paying for the legitimacy that being reviewed and published grants their work, and the journal still receives the publishing fee. And the journals will always get plenty of money flowing in from all of the universities that provide access to their students.
Who's really losing out here? Anyway, SciHub is a thing.
For people who care about, support, or build Free, Libre, and Open Source Software (FLOSS).