I see that the rewards for this program are for a limited time (extra 3GB for 2 years). Maybe they realized their free plan + referral space was too generous. I currently have 21.9GB for free on my Dropbox taking advantage of the 18GB of referrals you can get as well as the additional couple GB they give you if you use the "Camera Upload" feature.
If they gave all these students that 3GB permanently that might bring people up to some sweet spot where most of them wouldn't have a real need to pay for the service.