Ask HN: Is Copying HTML Bad and Unethical?

I'm wondering about this, as the result of recent Curebit/37 Signals Scenario. Which has 3 components to it, using design, Linking to resources and Copying HTML Source code including ids and classes.

In this question, I'm sticking to Copying HTML Source code issue only. I've seen Old School internet people talk about, copying other websites source code, and tweaking it a bit, to make what they wanted. I guess, that is one of the main reason HTML/CSS/JS is in the open for everybody.

Am I missing something? Lets say, If I copy complete HTML of a site, and change it with my content, is it bad? Does it count as plagiarism?