I get it. This is aimed at left-wing people who think their side can do no evil and thus proceed to censor any disagreement. But it's still funny to me that this whole thing needs to be preceded with 'the right is bad, but believe it or not, people on the left might do some bad things too', as if any side has a monopoly on virtue and goodness.
edit: This is a genuine question, not whataboutism. The letter is obviously very keyed to the current situation, rather than just a broad defense of free expression. Many of the signatories are presumably worried that backlash to perceived unchecked movements on the left will lead to the Trump administration retaining power. The parent commenter suggests that this kind of letter would be written in a vacuum, and I disagree.
Maybe a Dem POTUS rallying against cops as they try to stop protestors from confiscating and destroying various means of production?