Yes, Sallie Mae is doing this for a profit. Most likely they get a fee for each transaction. But then the business profits as well because it likely increases traffic to their stores. When you go to the grocery store or shop online, more likely than not you'll also buy something that you weren't originally intending to. That's how the stores recoup the cost and pay sallie mae for this service. For us, it's a tool. If used well it can have some benefits as long as it doesn't change our spending habits, like buy more than we would otherwise. When I shop online, I generally look first for the best price and then look to see if there are any upromise benefits. I'm not sure that I've ever noticed higher prices simply because I use Upromise.
Again, I've used it for several years, it hasn't made much difference in my loans but I guess it's better than nothing. I think it's more helpful if you have friends and family who sign up as well.
Other thing to consider, you are handing over a lot of your life when you sign up. They track your spending through loyalty cards and credit cards. Please keep that in mind when deciding.