Regarding copyright, copying for display purposes non-commercially seems to be fair use. There are four factors to fair use analysis: purpose (non-commercial art project), nature of the work (short factual public comments), amount taken (individual comments in context), and market harm (none, your comment still exists on HN and YC loses nothing). Three of the four factors favor fair use, and arguably all four.
The point is, it's not as cut and dry as you're portraying it to be. If you post data publicly on the internet, you should expect it to be read. The current legal system seems to support this theory.
Since this neither violates the CFAA nor copyright, it seems ok. Publicly posted content carries an implied license for reasonable access and display. That's why you can quote HN comments in a blog post without getting sued.