As you said, you "copy" the answer, you don't "steal" it.
How you steal (making a copy) doesn't affect the fact that you prevent the author to get his share or reward from it.
If you don't pay me, you're stealing from me.
Comparing this to your comment is whataboutism. Comments on hacker news are free to read. If you want to be payed for your writing, use a platform that support this.
However, you do have an automatic copyright on everything you write and I would need your permission (eventually by paying you a fee) to republish it in a book or somewhere else. Fair use is an exception to that, but the amount of text we can reuse without permission is limited. These are rules.