That would be because it is a form that is inherently damaging, so it looks like you don't have to "directly affect" the target itself, just create fire close enough to it for it to burn. But anyway, Mighty Torrent of Water and Crystal Dart don't have requisites for their target either.
Now, here, the spell is restricted to water-based targets, by relying on guidelines that make Ignem spells extra effective against Aquam targets. Adding a requisite feels wrong (and counter-productive), regardless of the wording of how the spell is supposed to work: the Auram version feels very contrived in my opinion.