Litigation heavy society + Governmental food safety guidelines.
Every menu in the us that has options for even slightly pink burgers, eggs, ceviche, sushi, or anything else not cooked to federal food safety minimums has that disclaimer on the menu.
We all know it, there's no reason for it other than perceived cheap lawsuit protection.
Thing is, if the food was stored negligently that message won't stop them from being liable. And if the food was stored properly, the chances of contamination are pretty damned low.