Why is a control surface that allows two sides of airflow better than those that allow one?
For low drag, you want to minimize the surface changes presented to the air flow. Large changes in the surface will create turbulence and pressure changes that contribute to drag. Small changes will minimize these sources of drag.

When you move a control surface like an aileron or elevator, air is flowing past both sides of the airfoil. Although this changes the direction of the flow, which gives the control force as well as some drag, the air can still flow past both sides.

In the case of a spoiler, the air cannot flow past both sides. This increases the frontal area presented to the flow, and creates a low pressure area behind the deployed surface. Creating a low-pressure area behind an object is a big contributor to drag, which is why aerodynamic bodies tend to have a teardrop shape.

Spoilers on the wing also have the effect of reducing lift. By disrupting the smooth low-pressure flow over the upper surface of the wing, they reduce the effectiveness of the wing, effectively stalling the affected portion.

For an illustration of both principles in action, see this video of a speed brake on a Mooney. This shows how even a relatively small object, when disrupting the air flow, can add a significant amount of drag.