and refuse to purchase from them on principal...can you elaborate on why?

I have recently heard many people (online and IRL) make comments along the lines of the following:

"I would never buy from an MLM or Direct Selling company"

Just wondering the logic here for writing off an entire type of company? Of course I understand products may not appeal to people - but what I am more intrigued by is the people who would not purchase from them just on principal...because they are a direct selling business...