In the light of development theories that show how the poor are doomed to remain poor under the status quo, is it morally correct for poor people to buy pirated copies of books? Original books cost around triple the price of the pirated one. If yes, how would one define 'poor'?
I would argue that it is never morally correct to steal. I think Kant's categorical imperative exemplifies the reasoning behind this belief. He says, "Act only in accordance with that maxim through which you can at the same time will that it become a universal law." By this he means (please correct me if I am wrong) that you should live according to those ideas which, if everyone adopted and implemented, would improve the general quality of life for society.
My point is, if everyone stole because they were poor (no money) then this would degrade trust among people, and society would breakdown. This idea may breakdown is extreme circumstances such as violent dictatorship, but I think its generally a good rule to follow.
if you use pirated copies you are infringing on people's rights. Infringing on people's rights is wrong. And being poor does not change the logic.
Is it morally excusable for poor people to buy pirated copies of books?
It depends on several factors.
I believe your question is whether it is morally excusable; not what should the poor to purchase an important book. If it were so, there are several possibilities according to the situation. Here the person who exploites is comparatively/morally worse than the buyer. So I treat your question this way....
Though poor people's purchase of pirated copies is excusable if it is for a good purpose, selling is not so excusable if the pirated-copy-sellers are making big profit from the pirated ones. Sometimes very rich people may be buying from them.
Another thing is, you say that the original book costs triple the price of the pirated one. That means the actual production cost is very less. So the authors are cheating people and this should be brought to light and make them humble to the society (only if the profit is by looting people).
Again, if the book is a vulgar one, the pirated-copy-seller is helping to propagate bad or wrong ideas. This is wrong indeed. If so, it is not excusable. They should be given the same punishment given to the authors. On the contrary, if it is a good book that leads people to become good, IMO, it is a praiseworthy deed.
There would be so many reasons behind every man's poverty. Among them the rules/laws of the past or present govt would be there. Most often it is man-made rules / laws that make people poor or rich. It is not because of our own effort we become rich. Actually if we ponder over the great work of nature we would never shove the poor and downtrodden in disdain. Basically as they are human beings, the poor also have equal rights to live on this earth.
Suppose 'A' conceive an idea and writes all the things for a book and B, C & D make effort to make it into printed form and E, the seller. Don't forget that through a book the writer is trying to spread a good idea. It must never be bad. A printed book is for the idea; not as a material. Even if B, C, D & E are not there, some other agency would do it for A...if the content is good. If A's objective is just spreading his good ideas, we cannot deny it if it is through pirated-copy-sellers. That is why I said it is praiseworthy. (You should connect this to the above para to get the apt meaning.)
My protest is against exploitation; not against making profit.
If the question were as I mentioned first, my answer would be different; without changing the above sentence.
Victor Hugo had said the same thing in his novel, https://en.wikipedia.org/wiki/Les_Mis%C3%A9rables
"Blessed are the merciful." This can't be wrong especially in this case.
how would one define 'poor'?
Let me ask a counter question.
Suppose a poor seller is selling something at moderate price. Two men (one poor and the other rich) approaches (not at the same time) for buying the thing from him. Who would you treat as poor......The poor man who readily buys the thing from that poor seller without bargaining? OR The rich man who buys after bargaining with that poor seller? Economics will certainly say that the first man indeed is poor. But the seller wouldn't say so. Can't we call those misers who behaves like this, poor? .... at least in some occasions?