The left never apologizes for the nuts on the left.
The right should not apologize for the nuts on the right.
Did the Unabomber hurt Al Gore?
Did Al Gore ever apologize for the Unabomber?
You're absolutely right, but the left has a cause that is popular, or popular with the press, at any rate, so that makes it OK.
In any case, we're better than they are! Violence is much more acceptable on the left; look at their "anti-globalization" riots.
Still, I think we have to avoid going overboard in our apologies; we, after all, did not encourage murder, do not condone it, and our principles do not permit it. So, in a sense, there's really nothing to apologize for.