In a constant speed propeller, why does the RPM drop with a prop pitch increase?
The propeller blades indeed have least resistance when aligned with the inflowing airstream, however the inflowing airstream is not the free stream at infinity, but the vector sum of free stream velocity $V$ and rotational velocity of the propeller blade $\omega \cdot r$.



In the picture the local Angle of Attack is indicated as $\alpha$. Increasing blade pitch results in increasing $\alpha$, which means more air accelerated backwards: more thrust delivered, and more torque required.