To take it to an extreme, it's like saying you murdered someone when you ran a red light and hit their car. Yes, they died, but you're only really guilty of involuntary manslaughter. Manslaughter gets you 5-10 years, murder gets you life. Two sides of the same coin, maybe, but wildly differing laws, punishment, and intent.
Stealing and pirating are both taking something that is not rightfully yours, but they are barely in the same category of crime.
We correct you because you're using misleading terminology, as cryptoz explained.
Language matters and it's hard to have a useful discussion when either side keeps on falling back to prejudiced terminology.