Not hiring someone based on political beliefs?

I’m in the process of applying for jobs. In the job ad it states that they will likely check out your social media, so be sure to not be to expressive of your opinions etc on social media.

Now I’m not talking about outright racism or things of that nature, but do you think that’s acceptable for an employer to do that? What if a candidate on their personal social pages outright express their support for trump? Or what political party they support? Or their stance on the Palestine situation etc?

At what stage does it impact freedom of speech? Is it like saying you can have freedom of speech but it’ll impact your chances of employment? If so, do we really have freedom of speech?