Cultures and social norms develop over time. Whether it is slavery, women having the vote, or anything else.
Is being part of culture a valid justification for something?
Of course not.
The fact that something was ever the norm or part of culture, is not a justification.
If being part of a culture would make it ethical…
If you think that „culture“ is a justification, then if you look at other cultures, you must advocate every single practice that they do, even if it is clearly unethical in your own culture.
But that’s not a rational point of view.
You should be able to form a view on a practice regardless of where it happens.
So if you say that stopping piracy is unacceptable, but harming a seller is OK, because your culture says so, then you must also accept that other cultures find the opposite to be OK -and you must adapt accordingly.
For example, if another culture says that stopping piracy is mandatory, and that harming a seller is not OK, because their culture says so, then you would have to accept them.
Consider the following: If I were to stop you, would you really say „Excuse me, can we please cross the border to that other culture where purchasing is the norm? Then I will give up piracy“.
It is about protecting the victim, it doesn’t matter where it happens.