You're right that people who get really sick shouldn't be jumping right into the Master-Cleanse without cleaning their diet first. People like that are too toxic and should do something to slowly detox like drinking fresh juices and eat raw veggies/fruits. Might apply to smokers, drinkers, coffee addicts and druggies too.
The average, typical American diet person doesn't have to though. A lot of people on here were eating proccessed foods, meats, fast food, junk food,etc right up untill the day before of the Master-Cleanse without anything more than the normal detox symptoms. (I'm just saying this because this question comes up all the time and that post might discourage newbies thinking they can't do it)