Because this way they can trade each other bitcoins for goods. If they just gave everyone mitcoins, they would be worthless for a while, and most of the students wouldn't care to even use them.
When I went to college, we could use our ID cards to pay for printing, meals in the student union, etc. If MIT did something similar and accepted "Mitcoins" for transactions done on campus, they would give the currency an inherrent value.
There is already a TechCash system for making purchases with an MIT student ID. All the on-campus food vendors accept it, as well as the student bar. No need for crypto-currency, there's already a functioning local system.
they could pin the value of the techcash currency to the USD/BTC rate