The only students that would require that would be students who make heavy use of MyMultis. That might be the norm at UNSW, but at every other uni in Sydney - not so much.
I dont think being "fair" is a point they have even considered - there are plenty of people not on concession who might be prevented from using Opal cards for the same reason.
There are students who first take the bus to a station then a train to Redfern or Central, that go to USYD or UTS. Taking the bus and going to USYD/UTS are not mutually exclusive. Likewise, not all UNSW students take the bus to uni - it might surprise you that around 40% of students don't take public transport to UNSW. Even if we only consider those students, you can't have half of those having to pay more (than usual) than the other half because bus services in their area don't have the card readers.
Those who pay full fare will still benefit, whether they use Opal or use their current MyMulti's or MyTrains - because of the cap in how much Opal will charge per week, whether or not they use Opal, they will still save compared to buying individual tickets. Yes, there may be customers who pay more than others, but they still save money overall.