I know what you mean. I've owned a couple of vehicles over the years where the fan didn't need to be on for the air to circulate when the control was turned to vent. I think they were made in the 60's and 70's though.
It does vent from the cowl when fan is off. I think when driving there is somewhat of a forced air induction into the cowl vent that creates a small draft thru the vents. To prevent this which I usually do cause I don't care for the draft I have fan off and recirculation switch on which illuminates.
I turned my A/C off (77 degrees here today) today while traveling 60 MPH and there was air flowing through the vents with nothing turned on. Not a lot of air circulating, but there was definitely air flow.
At this point, I'll have to admit that I have no idea what the problem actually is. Let's start over. If you press the A/C button, it turns the A/C compressor on, but only if the fan is turned on. If you press the button again, it turns the A/C compressor off, but the fan still blows air. The air the fan blows is either outside air, or recirculated air from inside the truck. That's determined by whether or not you press the recirculation button. But, when you turn the fan on, you should have air circulation inside the truck. If the fan is not turned on and you get up to a decent speed, you should be able to feel some air coming in. I don't understand why you don't just turn the fan on if you want some air in your face.
Fan off in dash vent position with recirculation button lit equals no draft. Fan off in dash vent position with recirculation button light off equals a draft. Of course draft increases/decreases with vehicle speed.