There are really two questions in one here:
1) How can they get away with it? (i.e. why isn't it a bad thing)
2) Why would they choose to do it? (i.e. why could it be a good thing?)
Regarding the first question:
These fighters primarily rely on elite level head movement and footwork rather than blocking or parrying to protect their head. In addition, they have the ring experience to know exactly when they can and can't get away with dropping their hands. (You'll notice that elite strikers don't leave their hands down all the time - they choose when to do so.)
Regarding the second question:
Having the hands down conserves energy and allows the arms to stay more relaxed. It allows the fighter to throw punches from angles that wouldn't be available from a tight, high guard. Sometimes it may allow for a more powerful punch from certain positions. Dropping the guard may be a deliberate ploy to induce the opponent to attack along a certain line. In MMA, the low hands can give a big advantage to stopping takedowns.
There are also less good reasons why non-elite strikers may drop their hands, i.e. fatigue or adrenaline.
One of the guys at my gym is a
two-time world champion boxer. He teaches a variation of the Philly Shell structure, which involves leaving the lead hand low and relying on the rear hand and the lead shoulder for blocking head shots. I've gotten to the point where I actually feel more comfortable with this as my default sparring structure.