Why have abilities that can scale with AD? It's not impossible, but it shifts how items are made around the concept. If no skills had AD scalings unless with an autoattack, then
would have much heavier stats on it, moreso than the heavy buffing that would be required of AD items across the board, because the stats on it wouldn't commonly cross-utilize champions (few champions could use both the AD and the CDR, as autoattack utility is limited).
Lethality would be a similar story, except on an item group that already has a bored out, limited niche. If this is a crack on Lethality items being more effective on ranged champions than melee, there's actually an easy way to fix that: increase the Lethality, drop the AD. ADCs naturally have lower AD values, so the spiked AD values on the items make it a HUGE improvement for them, and less for champions who rely on their base damages. The sad thing is that these items are supposed to be tailored to AD casters, but they couldn't be better suited for ADCs. Dropping AD on the items would be huge toward maintaining the intended audience. They've already done so for AP champions (MPen comes with somewhat minor amounts of AP, if any), but for some reason not so much on AD.