I mean i would understand if it did magic damage and scale with both ap and ad just like corki's rockets and q, but scaling just with ap and doing phyysical damage makes no sense, you get nothing by building ad, the only way your ult is making more damage is by leveling it up...
Bullet Time also deals magic damage based on Miss Fortune's AD, by virtue of applying Impure Shots. I think the point here is that Miss Fortune gets access to strong and consistent AoE burst, but that burst doesn't fall off if she falls behind or become too strong once she gets a full build. Marksmen are generally supposed to be all about single-target DPS, so giving one a bursty AoE nuke would also require a huge amount of control on that ability's damage. Having the major part of MF's ult scale with AP means there's little variance to the ability (you deal approximately the same amount of damage regardless of how far ahead or behind you are), and having it deal physical damage means her opponents can still counter the brunt of her damage through armor (so it's not an either-or situation where you either die to her burst or her damage over time).
Because they're bullets?
They don't want the abilities scaling to do more damage. If they scaled with AD they'd have to lower the base damage and scaling most likely to compensate. There's no reason for that.
So that it can scale off AD to deal more magic damage silly!
