You're right. Prices reveal true preference and constraints. The market is a machine for answering the question "What should we be working on?". Sometimes, people don't like the answers the market produces and invent justifications for distorting or ignoring price signals. In the limit, this practice amounts to a command economy. Disregarding price signals makes us all worse off and we shouldn't do it.
If something is expensive, that means it's scarce, and if it isn't, that means it isn't. If there's some negative externality you think the market isn't capturing, well, capture that externality. Put a number on it; don't make it a moral issue. It's only through numbers that we understand the world.