Quoting from their website, "The FSF is a charity with a worldwide mission to advance software freedom."
The question for the FSF Board isn't whether Stallman deserves to be punished, or anything - they're not a court of law. The question for the FSF Board is whether adding Stallman to the board accomplishes their mission of advancing software freedom.
The FSF is not the Stallman Foundation (the way that there's a Gates Foundation or a Chan-Zuckerberg Initiative or whatever), whose goal would be to support Stallman in his works and in making the changes he wants to see in the world. It has a specific mission, and the question isn't whether Stallman is so bad that he deserves to be excluded, it's whether associating themselves with Stallman helps them achieve that goal.
Given how much of the free software movement he has alienated, including past FSF staff, past FSF board members, and a number of present and former GNU contributors, as well as other organizations like Software Freedom Conservancy that work closely with the FSF and defend the GPL and software freedom, it seems very unlikely that it does.