Why is it okay to talk softly to someone else in the library, but not on your cell phone?

Ever in a library near a tutor teaching some kid grammar? Maybe they're talking low or moderate, so it is tolerable, right? Well, as soon as you hear a low voice talking around you, then realize it's someone talking on their phone, that immediately becomes rude or unacceptable. In both cases, all voices are audible, but the cell phone user is considered an inconsiderate jerk.....Why??