Agreed 100%... take for example the humble coffee card (you know, the every 8th coffee is on us) type cards. There are way's to work those, for example a $3 small cappuccino earns 1 stamp, but so does a large $5 mocha. Sometimes they state what sort of coffee is the free-be, sometimes they don't... So it's possible to fill the card using nothing but $3 small caps, and then order the $5 large mocha as the free-be.
It's also possible to "do a coffee run for the office" and collect the stamps on your own card. (I earn loads of free coffees that way), and this is using a pretty simple rewards program of "your 8th coffee is on us"...
Basically if you have a rewards program, you expect there will be a few who will find the loopholes or the angles. The reality is however that the average user of said reward program is not going to be devious. They are going to diligently collect the stamp for every coffee they order, and when it comes to the free-be, they will order the same as they have always done.