Why can Azir's soldiers proc Frevor of Battle's damage?

See title.

Frevor of Battle is specifically an on-hit ability, but his soldiers can apply the effect (not just gaining stacks, actually DOING damage). His soldiers aren't allowed to use any other on hit effects in the game (since they're technically not auto attacks), so why FoB?

It adds an extreme amount of free damage to his W, even with the upcoming nerf to FoB.