* For definitions of "Chinese government" that includes the Beijing non-profit China Internet Network Information Center, which isn't technically part of the government, but presumably is easily pressured.
* * For definitions of "all browsers" that excludes some minority browsers and those browsers run by users who have disabled the CNNIC root cert.
I have no doubt that the Chinese government has access to the CCNIC root certificate private key if it so chooses, but demanding the private key for an existing domain certificate would provide slightly less traceability and slightly more deniability.
If the encryption is then broken and it is done again, then a) it will prove that China did it. Because you can see who signed the certificate. b) it will prove that technical countermeasures are not enough, since the problem is deeper than that.
Because someone might like to mess with train signals, and in the off chance that some weakness is found in the MAC/signature scheme you're using, forcing an attacker to guess at which messages they're manipulating and how will make their attack more difficult.
If you're working with a networked application, even if it's on a non-public network, you should be asking yourself "why not encrypt?" instead of "why encrypt?". This is doubly true of critical infrastructure that's expensive and slow to replace.