You're incorrect. Read the entry I linked to above.
McDonald's was in fact negligent, and the case didn't "open the books for frivolent lawsuits far and wide" because her suit was not frivolous.
No, a room full of idiots decided they were negligent, there's a difference. Buying a hot drink and then being surprised that it is hot is pure stupidity. That's like buying a car and being surprised it can go over 30mph.