User blog comment:JmLyan/Ignita, the Disciple of Fire/@comment-1330314-20160428030712/@comment-7481207-20160428111114

Runeflare's shield scales with AP for the same reason other AD champions' shields do: it isn't meant to scale at all, but gives a small bonus for any occasional AP gotten from Baron/TF/New Guinsoo's etc. have been known to be problematic, especially with low cooldowns like Runeflare has. Its low base values (and non-scaling) is because of how frequently you can refresh it.

Her E actually has synergy with her ult, as it increases its movement speed bonus, but I see what you're getting at.