I think the core idea of open source is that code should be shared for free. Not shared for free (with people I politically agree with). By policing who can use software, the author's of this letter are very much going against the spirit of open source.
It should not be the position of the tool maker to dictate who can use the tool they create. Once you put it out in the world, people you don't agree with will use your software (if you're lucky enough to have users that is).
I see no difference in slamming Github for having a customer certain activists don't like vs the MPAA cracking down on DeCSS.