I'll start from the end: Companies get FREE advertising from YOU, and unless you get them an actual customer, they don't pay.
Effectively companies are “forcing” you to advertise for free (forced by your desire to "make easy money"), and then they pay you a commission if you get them a paying customer.
And you may fail to realise that you are already paying them by placing their ad in your page. Because in order for you to get money, you need to get them a click that ends in a paying customer. You could have pointed that customer to something you owned and theoretically you could have received the full value of their purchase. So losing that is a payment you make in order to "refer" a customer... Its crazy!