I don't condone flopping either, but staying down is probably a way to draw attention to the foul. No referee is going to award a foul or even a penalty if someone who suffered a legit foul gets up immediately after. This of course is no excuse to cry and whine and act like they're dying, I despise it as well, but I get the general idea of drawing attention to the foul.
Also, as someone pointed out, you can't judge the severity of the hurt, it's a classic case of "this is why we can't have nice things" or the entire classroom being punished for two-three bad kids misbeaving. Players will always suffer legit hits that genuinely hurt, but the divers will make them pass as "actors" as well.