WRT the aerosol can, the lid is there to ensure it doesn't accidently activate and spray all through your bag, where it becomes an inadvertant flammable hazard. Its a Dangerous Goods issue, not a security issue as such (but the security guys are also there to enforce the dangerous goods policies to some extent). The signs in Melbourne state that you have to take all aerosols out of your bag prior to screening, so they don't verbally ask you.
As to what is allowed and not allowed through security, there is some variance on what individual airports (And even shifts at the airport) seem to interpret as inappropriate for taking on board. All you have to do is smile and accept their ruling. If they say no to your tripod, then you just have to go back to check in and have it hold-stowed. My opinion is that you have been lucky to take it through as many times as you have, so consider yourself fortunate when they don't say no
As to the bashing, well, security aren't everywhere to prevent people getting attacked... look at shopping centres amongst thousands of other public places. The whole event would have been over in less than a minute or two once "landside" near the screening. In this case a pylon was used... could have been any non-fixed article really.
This was not a random attack... the perpetrators knew exactly who they were after. This does not make Australian airports unsafe, it wouldn't have mattered where they were, this attack could have happened.