As far as I have always understood the accomplice, the get-away driver in this case is as guilty as the person who pulls the trigger. I believe that is how the government sees it as well. Kathy Boudin got off easy. She should have received life in prison without the possibility of parole at least.
I'm okay with the ones who actually shot the police getting higher sentences. (And she may have cooperated with information and did a deal. She had GOOD lawyers, of course.)
The thing that has always puzzled me all these years was why her husband got such a higher sentence, since he was legally in the same position as she was. Indeed, it could easily be argued he did less: it was Kathy who got out of the truck and tried to persuade the police there was no problem.
We may not know all the facts, of course, for instance if she spilled the beans on the others later, cooperating with authorities. Something was different, very different sentences, 20 years for her and 75 years for him.