disclaimer:
the word "you" is not intended towards anyone in particular...
when you release the brake lever the pistons in the caliper retract slightly...this is common because of the different style rotors used... semi & full floaters.
full floaters can appear warped if pressed the right way... the brakes have to accommodate after market rotors that people (racers) will (most likely) replace the stock ones with.
these "tolerances" can create some unintended nuisances.
enough of a gap up/down & in/out also helps with keeping debris/dust/rocks and other items from getting trapped and causing a fatal error.
notice how little pressure on the lever it takes to apply friction to the rotor...
this extra space also helps keep the rotors from prematurely wearing the pad in an uneven way.
as for anyone "flipping" their ride because of the pads moving...they are clueless. When traveling "at speed", and the brakes are applied...the pad is immediately drawn to the top of the caliper due to the rotation of the wheel. Besides, the distance the pad "might" move is nearly 1/16th ~1/8th of an inch overall (top to bottom).
it's not dangerous...nor will it adversely affect your braking....in fact, if you can hear it while riding, then you need to put on a helmet...or an exhaust, because you are aren't paying attention to the important things around you that could kill you.