From the top: the goal of the lockdown mode is to prevent someone who has UID=0 ("root") in the kernel from being able to modify or tamper with it (ring0 / CPL0 access). This can be done through /dev/mem, kexec, or even by asking a PCI card or FireWire device to DMA into physical system memory. Basically, there are a lot of hooks to tamper with the kernel, and "lockdown" mode turns them off.

Now, this only really makes sense if you are in some sort of trusted untampered environment already: if not, someone with root can stomp a new kernel in /boot/ which gives them a backdoor. So, lockdown without a trusted boot environment is security theater, since you can just take the long road anyway.

Additionally, in a trusted boot environment, the lockdown flag is wanted. If not, "trusted boot" really isn't such: someone that booted through a signed kernel can simply tamper with the kernel anyway after booting up through an init hook. The dangerous part: once they have the ability to tamper with the kernel, they can chainload another operating system and functional as a poisonous bootloader. The danger here is that if an attack like this is seen in the wild, and the attacker used, say, Fedora's kernel signing keys to do the attack, it has the possibility to get Fedora's keys blacklisted by OEMs. Linux would do best not to be used by an attacker as a "malware bootloader".

That means it's reasonable for, by default, lockdown to be informed by the presence of a trusted boot environment.