Why? She obviously doesn't mind getting punched out, so why should it be any of the NFL's business?
For clarity's sake, the them I was referring to was the Ravens. So... to answer the question (the answer would be pretty similar if I had been referring to the NFL instead of the Ravens)... because he is their employee. The Ravens have an interest in what kind of a person he is, not necessarily what kind of a person she is. They have an interest in who he is and what he does that exists without regard to what she thinks of who he is or what he does.
Just because a bank forgives someone that robs them, that doesn't mean that the employer of that bank robber has no interest in the reality that they are a bank robber. Maybe it's McDonald's or Ford Motor Company, regardless they might care that the person they employ is a bank robber and they might not want to employ that person anymore. Maybe they're a landscaping company, they still might not want to employ a bank robber. A rape victim may forgive their rapist, but other people in that rapist's life still have an interest in what kind of a person they are - they still might not want to have anything to do with them as a result, an employer might not want to continue employing them as a result. That's true for all kinds of reasons, many of which I'm sure you understand without their having to be explored. The point being, from the Ravens standpoint this is about who he is, not who she is. Whatever she is or does doesn't fix the defect in him. He's a POS no matter how much he loves her, or how scared of him she is, or how much she's willing to put up with.
Further, and to make a broader point, part of the problem with man on woman violence is that other males don't often enough call their male friends or associates out on it. That's part of why it continues, or at least why individual abusers can go on abusing. As men, I think we have a social responsibility to call it out when we see it with people we know. It needs to be stigmatized to a greater degree than it is - it needs to be seen as unmanly, as punk-ish, not just as a criminal lack of temper control; it needs to be seen as the indication of male insecurity that it typically is. We should be saying - hey ####tard, that ain't cool, men don't hit women, only insecure little bitches hit women. And the Ravens are an organization full of men. I'd bet the people that made this decision, at least some of them, are men. As such they should call him out. They probably should have done it a while ago, but at least they're doing it now. They're saying - this isn't okay and we don't want to have anything to do with you anymore. A business organization built on the notion of masculinity, that celebrates barely-bridled male force and aggression, is telling him to #### off - because he hit a woman and knocked her cold. That's a start, that's the right kind of message to have sent. In a more ideal world they'd feel free (and maybe obliged) to beat him down to get the message across, but our society tends to frown on such things.
Anyway... good on them for what little they did or felt they could do. How a female victim responds is one thing - sometimes it's lamentable, even if understandable - but how other men respond is another. And males that hit women deserve to have men call them out on it. That calling out can take many forms, in this case it was a firing which will, among other things, likely cost him a lot of money.