The reason the indicated speed stays the same is that the air mass is moving and you are moving along inside it. Its easier for many people to visualize a boat on a river.
If the boat is moving through the water at 10 knots, it doesn't matter to the boat whether its going upstream, downstream, or across the river. It is moving through the water at 10 knots. Now if the water is also moving at 5 knots, then the boat is effectively moving 5 knots upstream and 15 knots downstream COMPARED TO
THE STATIONARY LAND. To the boat it will still be 10 knots.
Same thing for airplanes. The speed of the plane through the air stays the same, regardless of the wind. But compared to the ground, it will be moving faster or slower due to the entire air mass moving. Much the same was as you are really going thousands of miles an hour right now - around the sun that is.
And be careful with your terms... "True airspeed" or TAS
is indicated airspeed adjusted for altitude and temperature. "Indicated airspeed" or IAS is what the instrument reads.