slight complaint(more of issue?) on Tristana's R

So: Buster Shot Active: fires a massive cannonball at an enemy unit, dealing magic damage and knocking them back and surrounding units.

Massive canonball. let that sink in. Massive ball of [insert hard substance of choice]

How does a 100% physical damage sounding skill deal magic damage, and no physical damage and have an AP modifier and not an ad modifier?

also, if u get hit with enough force and hit back through your team(one being hit back's allies) how do they take no damage from you flying through them?

reading about new trist kit got me thinking this way.

W being AP sounds normal(rockets could burn u) E sounds like tiny ad (initial impact) and a larger ap (explosion)

R just makes no sense to me being ap and not ad :/