But there are plenty of people who are widely believed to have committed terrorist acts without being convicted... Wouldn't you call those people terrorists?

What about pejorative adjectives describing conduct that isn't actually illegal, such as shouting at restaurant wait staff over perceived mistakes? Am I allowed to call that person a "jerk"?

It would be plainly ridiculous to insist that we can only apply adjectives upon conviction by a criminal court. That's not how adjectives work.