Well, the owner/manager decides, but I guess I don't see the slope (in her case at least) as all that slippery. If you work for Planned Parenthood and tell people coming in that abortion is wrong, then don't expect to keep your job. If you work for the NRA and advocate more gun control, you're probably going to be in the unemployment line. I think it's mostly common sense.
She worked for Glen Beck's network, which has always been very vigorously pro-choice, and she essentially said that pro-choice conservatives were hypocrites. Not hard to figure out that wouldn't go over well. Beck is claiming that isn't the reason she was suspended, and it may just be that the backlash is temporarily toxic.
But look...the difference to me is if people are going after ordinary people because of their views, simply based on the idea that people with those views should be marginalized in general. Which means not just that you're going to lose one job, but that they want to keep people with those ideas from getting any job.