I wonder if the US authorities have the legal right to refuse entry and prevent a US citizen from returning home?
In this case, it isn't relevant, as he is a Liberian visitor, not an American.
However, in the case of certain diseases, you can be refused travel. That is true even of non-disease issues (such as cancelling flights into the US as a result of 9/11). So, if you got Ebola in Liberia and were just a normal citizen (not a politician, or a member of some religious group that will spend a million dollars on you, or an employee of a news agency that would get crucified in the media if they didn't get you back into the US for treatment -- or aren't part of any other special network that will provide you with a private jet and a biohazard team), you'd be stuck.
The only case where it would be a question is if a US citizen is bleeding from his orifices and wants to walk across the border at some point where they actually have border protocols (a Canadian checkpoint, say). I'm not sure if you can be precluded from entry, but my guess is that for those cases (which are so rare as to be hypothetical), you'd be held there until a biohazard medical team shows up. You can certainly be held at the checkpoint if you are suspected of various other things (carrying too much cash, having precluded food items, etc.).