The Discover Card does something similar: for periods of a few months they offer special rewards for special categories of products - for one period it might be groceries, for another period it will be gasoline, for another period it may be home repair stores, etc. However, to get the reward, I need to login to the Discover web site and make a couple clicks to get the reward. I do find it irritating and don't always remember to do it.
I think they do this for a variety of reasons:
As already mentioned in mhoran_psprep's answer, the card provider can save money if people don't sign up, while continuing to advertise that they give a 5% rebate (or whatever benefit).
Every time I log in I get asked questions which have the potential of helping them. For example, for the time being we prefer to receive paper bills, but every time I login to the Discover web site I am asked to switch to electronic billing.
Their web site also serves up advertisements for their other products and services, and sometimes those of their "partners" (companies they team with to provide some of these discounts, rebates, etc.). Exposing customers to these ads makes it more likely that those customers will use those products and services. Thus it benefits the company to have their customers login to the web site every month or so.