The fact is that what you do reflects on your employer to a degree. If you're a developer evangelist, that's quite a high degree, particularly if you drag them into your public spat.
Most people in the media (news readers and so forth) have a "morals clause" in their contract because they publicly represent the company. This situation is much the same.
I, for example, am just a software engineer. I pretty much stay away from any thread involving Google directly as I don't want anything I say to be misconstrued as representing the company's view. We have a media relations department for that. Those defending Ms Richards might say "on your personal time you can do what you want" (to me). Well, yes and no. It's certainly at my peril. Also, in Ms Richard's case, at PyCon she was on official (or at least semi-official) duties so the "personal time" argument doesn't even apply.
This whole incident reads like two guys making jokes to themselves. She overheard and decided to make an issue of it by naming and shaming them in a very public way while waving her employer's flag. Nevermind that this wasn't called for (IMHO), I don't believe for a second that the comments were directed at her or intended for her to hear.
The fact that Playhaven fired this engineer over this is disappointing. A public statement about supporting diversity and a warning from HR really should've been sufficient (again, IMHO). It makes me wonder if they wanted to fire him anyway.
Some other commenters suggest Ms Richards can sue. I tend to disagree but IANAL. I do however think the fired engineer may have a case. He can show damages (losing his job) and argue that Ms Richards (and her employer had they not fired her) had acted with reckless disregard. I'm not sure it's a strong case but I bet it's an arguable case.
Sendgrid's statement is (IMHO) excellent. It is measured, factual and has a great tone. This is not a position they wanted to be in (from reading the post). They've deliberated. They state a great bottom line that this issue and the subsequent fallout endangers their business. They don't want to be the story here and Ms Richards made them the story. That's on her.
Let this be a lesson to all reading this: what you do can affect your employment. Act with care and restraint. Once something is said or done it can be very difficult to roll it back.