Logic - there is none!
Personally I couldn't care if there are hornets over Melbourne or not. If they were not patrolling Melbourne then they would just be flying mindlessly up and down the coast off NSW doing some other "specialist training" that actually will never be required in combat in the real world. So the state that they choose to fly over while they burn up tax payer dollars in not important, that money would be wasted regardless.
The fact is that if one was determined to use some sort of aircraft to attack a city then one would be just as happy to drive that hijacked aircraft into downtown Cairns, Perth, Darwin etc etc etc
The fact that some sort of athletics games are on is not a set pre-requisite for a major attack is it?? I don't think that terrorists choose to attack the most heavily defended locations just to make a point. Their point gets across very well regardless because the media would be just as happy to cover a major attack on Townsville or anywhere else. I mean, having a few fighters sent to Melbourne might be a fun posting for the crews for a few weeks but it is not going to prevent an attack on anywhere else is it?