Our sailing wasn't as recent as some PPs, but we sailed the Fantasy in October 2014 right in the middle of the Ebola-in-the-US panic. Cabanas/soda machines/ice cream etc were all CM-served for the first 3 days of the cruise, literally starting with the embarkation buffet, I'm assuming because of this panic (though I'll admit I don't know for sure, it's the only explanation that makes sense to me seeing as we were a whole new set of passengers). By Day 4 everything had reverted back to self-serve. However on Day 6, we went to Palo Brunch and yes, a CM walked us through the buffet. This actually happened every time we returned to the buffet, but maybe that was on us for not declining the service on repeat buffet trips.
Considering the extreme health precautions that were in place at the beginning of the cruise, and then removed, coupled with the fact that I haven't seen any reports of Dream/Fantasy cruisers since that time where being walked through the buffet did not happen to them, some kind of infection makes no sense in our experience (unless Ebola somehow spreads through almond croissants?

). I would agree with the food wastage and/or some demonstration of service - the second to me seems more plausible since it's clearly not being implemented on the classic ships with their different layout.
For the record, this was our first cruise and we severely disliked this practice - my DH especially felt very self conscious, since he ended up not selecting anything from the buffet once he was able to see what was there, while his particular server was very insistent he just had to try something. It's certainly not enough to keep us away, but we do wish it wasn't there.