I'm not defending or rejecting Amway...You can form your own opinion there but it is not a pyramid scheme...rather multilevel marketing, in that all money (commision...bonuses....whatever) flows from the sale of products not the "Initiation fees" of new recruits. Just like conventional marketing and sales...the more the volume of sales the greater the profits..total dollars earned even on shrinking margins.
Many businesses are marketed this way...Some more ethical than others. MLM however is not an illegitimate form of business. It works well for many people, some of whom I know personally, I however am not right for it.
Here is an interesting link on the subject.
http://www.ftc.gov/speeches/other/dvimf16.htm