How does a contractor determine the water performance rating needed for a sliding glass door?
Generally speaking, there is no formal guide to choosing a door. However, a thoughtful contractor would have considered the potential for heavy rain and wind-driven rain (especially the direction weather comes from) when choosing a door. That said, it sounds like you did approve the materials, so this is partly on you.

Short of replacing the sill (which is tough), my best suggestion is to attach an additional piece at the bottom of the door that will slow driven rain down. This can either be a drip cap, a sweep, or a combination. Look at google images for "CR Laurence Door Rain Drips", for example.

Note that there might be factors that haven't come up that drove the choice: some homes don't allow a sill like that to be dropped enough that the lip at the interior isn't a trip hazard. Cost/timing may have played a role (ie, high lip = special order = $$$/slow delivery).