I think it depends on your location. In the US, I think there is little to stop people being fired for any number of reasons including, "I don't think you fit in". In most of Europe, being fired is very rare because it usually requires a major malicious or negligent act and even then, you are more likely to be warned first.

If someone simply isn't working well enough for their role, there is usually some kind of capability process after which their employment might be terminated but we wouldn't usually call that being fired and if that is made clear in the job description and in regular feedback, I don't think most people would consider that unreasonable.