The 'must use' scenarios for a bottom pad needing soldering are for 1) high power applications for heat-sinking, 2) RF applications which need a solid electrical GND connection and 3) high current applications where the pad is one of the electrodes.
This device is low power, low frequency, low current, so meets none of those.
Why does the package have a large pad?
Perhaps it's a standard package, and it would cost more to design a package without the exposed pad, than it costs to use it, if indeed it is more expensive than a package with no pad. Bear in mind the way many ICs are made, with a metal frame that defines the pins, which are then separated later in the process. Leaving the pad part of the frame in place may well be cheaper than removing it.
This is a very thin package, intended no doubt for space constrained application like smartphones. Any insulation or mold material over the exposed pad on the bottom would make the package thicker.
I'm intrigued by their 10,000g shock specification. If I was designing with it, I might be inclined to ask them whether soldering the pad down has any influence on that spec.