When you ride, you're creating wind (relative to you). If the air is perfectly still, and you're riding at 15 mph, then you feel as if there's a 15 mph wind. Aerodynamically, the same forces apply if you were perfectly still and there was a 15 mph headwind.
Likewise, if you have a 15mph headwind, and you're also traveling at 15mph, then the aerodynamic effects are the same as if you were traveling 30mph in still air.
Aerodynamic drag is proportional to the square of your velocity, so drafting really only helps at all if you're above 15 mph.