In some of the telemarketing areas, I'm sure the workers are very exploited. However, you need to take in the rest of the folks that are working on commissions only. For example, real estate agents. For those who have "their act together" - they are very successful. The very successful agents have paid staff members that provide a lot of support for their sales network. With all of this in place, they are able to move millions of dollars worth of property each year.
In closing, I have a friend that is involved with the sales of business machines and business solutions with a very large, well-known company. He has a process in place where he makes very good records of his sales because he gets screwed on nearly every commision check that they cut for him. He then digs out his records and provides the "proof" to them and then receives the correct amount for his commissions.
Sales can be a very tough role. You really need to deal with a lot of rejection. If you feel you are very good at selling items and have 6 or 8 months of "cash" so you can survive until the money starts to roll in, you will be very successful - even if it is a commision-only position.
Telemarketers ... ugh! I can't believe someone would stoop so low to do such work. I couldn't imagine how high the turnover rates are. Then again, the slummy companies probably don't care. As long as "x" calls are getting made on a daily basis, the are confident the word is getting out. The commissions in these roles do exist, but by the time a sale is eventually made down the road, I'm sure the telemarketer has already quit and left the company - so there is no way to receive the fruits of their work.