If the startup scripts are upstreamed and standard then the init system should be standard. Systemd is not standard because it relies on Linux kernel specific semantics.
BSD or any other kernels are left with no updated userland because they don't implement the same semantics.
You realistically expect BSD to ever use a GPL licensed init system? I don't see the point of systemd being able to run under BSD. BSD wants to get rid of every GPL licensed software, including GCC. You really expect them to consider switching their init system.
Aside from this, standard has nothing to do with Linux only or not. E.g. Microsoft Office is a de facto standard, it only runs on Windows and a not exactly the same version is available on Mac OS. Not on Linux, not on BSD. The file format is a standard.
Firstly, SystemD is licensed under the LGPL so it's not as viral.
Secondly, they've already changed project licenses before in 2012. Should the need arise to switch over to a more permissive license, they could ask again and strip out the code the can't get permission to change.
Perhaps it's for the same reason there would be no (FOSS) Intel or Radeon support, due to the FreeBSD kernel lacking features needed by those drivers.
However in FB9 they did get around to supporting KMS and are in the process of porting the latter over with the former already working.
My only concern would be if the developers flat out said no about going with a more permissive license, although I don't really see why they would do so as it is a program with limited applications outside of its intended role.
BSD or any other kernels are left with no updated userland because they don't implement the same semantics.