From the other side they see it as "You've obtained something that you did not pay for." Which is also a legit mindset.
It's not necessarily legitimate.
For example, I'd love it if everyone who flies in a plane over my house paid me, and at one time that might have been the law[1]. I could argue that they'd obtained something ("flying in my airspace") without paying. Whether or not that is a legitimate mindset is up for discussion. It may depend on other things like democracy and the public good.
You don't get to just pick on one side. I could equally well say that "You have not lost anything by me copying this software." is not necessarily legitimate.
I personally know many examples of people who have pirated things they would have paid for, had they not been able to successfully pirate it. You can therefore I believe argue that the piracy lead directly to a loss.
It's not necessarily legitimate.
For example, I'd love it if everyone who flies in a plane over my house paid me, and at one time that might have been the law[1]. I could argue that they'd obtained something ("flying in my airspace") without paying. Whether or not that is a legitimate mindset is up for discussion. It may depend on other things like democracy and the public good.
[1] https://en.wikipedia.org/wiki/Air_rights