Ok, let me explain how this works.
Yes, the energy suppliers make money by signing up people on the low rate plans fixed for several months, and then charging variable prices. Yes, depending on how greedy your supplier is, it may charge same price as utility, slightly more, or way more.
But, the suppliers won't necessarily screw up the cancellation on purpose. In most cases people just wait too long to cancel.
With NRG promotion they want you to stay 2 billing cycles (months) with them. The trick is to call to cancel immediately after the first bill with NRG as supplier arrives.
The electricity supply cancellation is never effective immediately. It will happen on the next meter reading date. So by cancelling after the first month, you will stay with NRG for the second month for sure. So you will still be entitled to your points, and will not spend extra month with NRG on variable rate.
If you do not call to cancel immediately after the first bill arrives, your cancellation might not be effective on the next meter reading date. And you will stay with NRG for one month more than necessary.
How quick you should call? For ComEd, you should call to cancel at least 18 calendar days before your next meter reading date. Other utilities have different rules. So you really have about 10-12 days to call after getting the first bill.
If you call after the points post, then you will for sure stay with NRG for one extra month, and, depending on how quickly the points post, may spend the second extra month with them.
If you still get screwed up, complain to the public utilities commission in your state. They are usually very effective at dealing with the suppliers.
Oh, and you don't necessarily want to cancel. You can call NRG and ask them the new fixed rate for the next 6, 12, or even 24 months. That rate may be actually good (same or better as your utility's).