Perhaps there's some confusion in the terminology? At least for the Space Shuttle Main Engine, the Augmented Spark Igniters didn't fire for the whole burn.
The igniters turn off after 4.4 seconds while the ignition
flame continues in order to prevent intermittent and possibly
damaging blowback from the main combustion area. This also keeps the
system simple and reliable.
This means the electrical igniters turn off, but propellants are still supplied to their little combustion chambers, and they continue to burn throughout engine operation. Valving off this small amount of flow would add un-needed complexity, weight, and failure modes.