Just noticed this today and it I can't really come up with a good reason. So, Does anyone know why the drivers side wiper has a "black plastic thing" underneath it and the passenger side does not? Looks like an aerodynamic shaped black plastic thing underneath the wiper arm.

 

I think it's designed to help ensure that the wiper has a strong contact with the glass - the additional wind force generated by pushing on the wing achieves that. I guess they decided it was only truly important for the driver's side, and only did it on that side as a cost-cutting measure.

At least, that's what I've always figured (same thing on 1st Gen MINIs too).

 

my 95 Miata was set up this way too ... deflector only on the driver side