There are 2 ways of looking at why they wore their mask all the time. They probably were one of those extremely hygienic med people who wanted to be extra careful. With swine flu in the air, there are people walking with masks even on roads so I won't blame these two if they were trying to take some precaution.
Another reason why they did not take their mask off could be that they do not have great oral hygiene. I am not but people are judgemental. When you go to the gym, you would look at how muscular your instructor is to judge how good a job he is going to do on you. Go to a salon and you are bound to judge the hair dresser's hairstyle. These two probably did not have great oral hygiene and in these tough times, no one would want to lose any business because of judgemental human nature.
I hope it is the former. I still have 2 more appointments with this dentist over the next one month and they better do a good job.
On a deeper note, people do wear mask all the time but forget that other people are people and judge them. Think about the last time you wore a mask and thought no one would notice it is not the real you. Think about the last time you judged someone for hiding their real identity behind a mask they thought you wouldn't notice. Think!