Exactly! When you download a copy of an article, there's an infinite number of copies available, and taking one of those copies almost never impacts someone else who wants one.
Many Sci-Hub users believe it is ethical to download copies of journal articles there, because they find the consequences of not paying for the article to be more beneficial to society than the consequences of paying: https://en.wikipedia.org/wiki/Consequentialism
In their view, closed access journal publishers are stifling academic discourse with extortionate fees (especially considering that they do not financially compensate the authors and reviewers), which means it would be most ethical to limit the closed access journal publishers' revenue in order to reduce their power and importance, and disrupt their business model to pave the way for open access journals. Not paying publishers for copies of articles is how Sci-Hub users help achieve this.
Many Sci-Hub users believe it is ethical to download copies of journal articles there, because they find the consequences of not paying for the article to be more beneficial to society than the consequences of paying: https://en.wikipedia.org/wiki/Consequentialism
In their view, closed access journal publishers are stifling academic discourse with extortionate fees (especially considering that they do not financially compensate the authors and reviewers), which means it would be most ethical to limit the closed access journal publishers' revenue in order to reduce their power and importance, and disrupt their business model to pave the way for open access journals. Not paying publishers for copies of articles is how Sci-Hub users help achieve this.