This is the same practice for the other private lift companies like Mitsubishi, Hitachi, etc. Each will want to protect their own turfs, not to mention the quality of service.I think you need to differentiate those that hate the government with no or little basis and those that have repeatedly questioned the Government conduct over numerous issues. Imagine a HDB owned company refusing to provide spares parts for lifts to private contractors who service lifts in numerous HDB wards.
In addition, should it be opened to everyone, it may lead to misuse. Take the CHAS scheme as a good example. Many private GPs and dental clinics had taken advantage of the CHAS scheme by overcharging their patients and/or providing unnecessary treatments so that they can claim even more from the government. Even when asked for an itemized bill, they will tell you that their system cannot do so. What a crap? Sinkies are dishonest and selfish, no matter in what trade. Now that the government has woken up and realize this loophole, they have started to crackdown on these abuses.