They still have copyright on the software: copying it and, say undercutting the AEC on the fee-for-service would be illegal. Far more likely that the AEC don't want to have their software open to scrutiny by politically-motivated geeks.
"Under the Fair Work (Registered Organisations) Act 2009 (the Act), the AEC must conduct all elections for office in registered organisations unless an exemption has been granted by the Fair Work Australia." http://election.aec.gov.au/About_AEC/AEC_Services/Industrial...
