Criminality has been dependent on internal mental state since the dawn of time. Like when you kill a person, it ranges from self defense to manslaughter to murder (and maybe terrorism now). That is not what "thought crime" means.
"Thought crime" is a poor description because it is not a thought being punished, but an action. You couldn't be punished just for thinking about stolen goods.
Analyzing intent is important to differentiate between people who have likely already learned from their mistake, and those who are likely to exploit others in the future. Punishment is only necessary in the latter case.
That's actually the only reasonable definition of crime in a perfect world. It's because we don't live in a perfect world that we must focus on acts instead of intentions.
It's been an important part of law for a long time, although there's a tendency to explicitly avoid it in legislative efforts in the US since the '60s and the advent of MPC (https://en.wikipedia.org/wiki/Model_Penal_Code)
