The systemd devs claim it has nothing to do with it, but with a bunch of other software that expects /usr to be mounted at boot time: https://wiki.freedesktop.org/www/Software/systemd/separate-u...

That's why you mount /usr at boot time... in your initscript...

Additionally, I think a good argument could be made that anything that systemd requires to boot should be in sbin, not specifically because systemd requires it, but because it's required or useful for the boot process, as systemd has illustrated, and thus should not be relegated to /usr (depending on where you draw the line between useful and required and whether useful is sufficient to be included in /).