Parking spaces are not free. In city's they generally cost 3+ $ a day in upkeep and opportunity costs.
Also, collages accept students younger than 18 which are not adults and legally can't be treated as adults.
I'm very interested in the upkeep cost - do you have a source for that?
"A 1996 survey found that commercial parking operating expenses average about $500" per year. Construction costs Dallas $13,281 to New York $20,326 av $15,552. http://www.vtpi.org/tca/tca0504.pdf
However, opportunity costs bump that as for example it lowers the density of the area.
Second statement needs a source, and regardless it isn't a valid reason. Colleges don't decide how adults live their lives.