GPLv3 is a very restrictive license, especially for what is essentially a micro blog (though I dislike the license for most open source software anyway).
Add on the original author going after a bit of CSS, not even the main effort of the project in question, and you've got my "petty" comment.
1. I wouldn't use GPLv3
2. I wouldn't care if people stole my code that I open sourced or if they tried to license it a different way.
3. I personally follow the license of others when using their code. I wouldn't steal GPLv3 code without proper attribution etc. That's their right.
All that doesn't go against my initial opinion: GPLv3 for a small micro blog templating system is lame. Enforcing it for a bit of CSS is petty.
It is a matter of principle, probably?