It's not an unreasonable thing to ask but odds are they just aren't thinking about it, so it does become up to you to make them think about it each time they forget, if you want to be sure to get all the information they're discussing. I doubt there is any disrespect meant; it's just not something hearing people are used to considering, sadly.
Remind them as soon as they turn around - and do it every time until they get in the habit. If you're kind but firm about it ("I'm sorry, I know it's a change in habits, but I really cannot understand you if I can't see your face. Please turn around." "I'm sorry, I didn't catch that - can you repeat everything you said while not facing me? As I mentioned, I can't understand you if I can't see your face, and I don't want to miss anything important." ) they should get the point with no harm done. And if they DO get annoyed or frustrated, well, THAT is definitely NOT your problem!