It's not encroaching on fstab, it's using it as input during boot, because mounting is intimately coupled with service ordering. Think about services that resid

It's not encroaching on fstab, it's using it as input during boot, because mounting is intimately coupled with service ordering. Think about services that reside on a network filesystem: the network has to be up to mount the filesystem, but the network in turn can require arbitrary services to run, which can require other local filesystems to be mounted. You can't get configurations like these to work without coupling mounting and service ordering. It's fundamentally not that different from sysvinit, with its various init scripts that mounted filesystems according to the fstab.