Why is Steraks considered bonus ad instead of total ad?

You mean base ad, instead of bonus, right?

The goal is to very specifically statbind the item so it's much more difficult for champions not meant to use it to also abuse it's offensive potential. For example, assassins and dive fighters tend to have a lot of bonus ad ratios but few if any total ad ratios. This means that even if you buy sterak's for its passive on an assassin or dive fighter, they aren't getting as massive a damage spike from it as well.