Thanks!

With cheese you add an acid or lactic acid bacteria – this brings down the pH (makes the solution more acid) and two things happen: the milk curdles so you can turn it into cheese (or yoghurt) and the lower pH makes it less hospitable to other bacteria, thus preserving it. (Grossly oversimplified – but you can make ricotta at home with milk, salt and citric acid).
With beer the yeast digests the sugar into alcohol (which is a much nicer waste product than lactic acid!) – alcohol is v good at killing other microorganisms too.
Safe? There may be some potentially harmful bugs (Listeria in cheese springs to mind) that can tolerate the pH in some cheeses but during most cheesemaking processes added salt plus dehydration tend to favour the nice bugs and not the nasty ones. And if your immune system is working you can tolerate quite an assault from Our Microbial Overlords. Blessed are the cheesemakers, innit.