And bash isn't what is interpreting the script?

Yes. Init scripts are always interpreted by /bin/sh, even if the default $SHELL is /bin/bash.

qwurx [shmem] ~> /bin/bash -c 'if [[ 1 -eq 1 ]]; then echo yup; fi'
yup
qwurx [shmem] ~> /bin/sh -c 'if [[ 1 -eq 1 ]]; then echo yup; fi'
/bin/sh: 1: [[: not found
[download]

update: not sure what might befall within the systemd usurpation... *sigh*