Ethics are tricky, and I don't think there are blanket wrong or right answers here. I agree that simply placating a baying mob by tossing someone (metaphorically) to the wolves is not necessarily ethically right from all perspectives, but therein lies the problem: it's about perspective. Were it about a university defending an author from religious nutjobs then it's ethically right for them to support him. However, when it comes to companies, there are certain other perspectives on ethics that need to be taken into account. For example, ethically a company has to do everything it can for it's shareholders. It's legally required to do this, and has an ethical responsibility to. This might be in contrast to yours or my ethics, but for the company the ethical thing to do would be to fire someone who's losing them money.
Anyway, like I said, it's tricky. Personally I think SendGrid is justified in firing Adria: as a PR employee her job is to promote the company, which is difficult when the community she's trying to promote the product to has taken umbrage with her. If their sole reason was because of the DDoS attack I think that wouldn't be justified, but given the fiasco it's damaging to their reputation to continue to have her on their staff.
As an aside (and to hopefully help explain my viewpoint) in the Dawkins matter: I don't think it would be ethically irresponsible for a publisher to drop him. Publisher's are there to make money, and having that and the lives of employees threatened is a reasonable reason to drop him. I don't think they should drop him, but in my opinion it's ethically reasonable for them to do it. Were it a university, I think they under no circumstance should drop him. The purpose of a university (at least in the UK) is to teach and generate research. By censoring someone at a university you're essentially censoring science and scientific enquiry which holds much greater ethical ramifications than at a publisher.