it's probably the wires keeping it up, lol. i had 3 different windshileds in my rodeo, and each time the rear view rattled completely off. twice, taking the glass to which it was bonded with it. if it's not an SUV, i believe it will be illegal to remove it.
yeah, i'm pretty sure its illegal to remove it. if its the base that rattles then you can usually dosomething to tighten it up. if its the glass, theres not much you can do since you usually can't get the glass out without damage
theres either a screw that you can tighten or it needs to be re-glued.. try a thin layer of foam glued between the mirror and the plastic on the windshield