"Free" Weight Loss Trial

So, I am driving Bubs to school this morning when the following commercial comes on the radio...it went a little like this: 

 If you have more than 20 pounds you want to lose and are not enrolled in any other weight loss program, you qualify to be part of our FREE weight loss trial.  Weight loss is guaranteed or else you do not pay for your trial.

 Huh?  Why would you pay for a free trial in the first place?