Why is the SDK free?

Because the asset is the evidence model, not the software. A model that is not widely instantiated is worth nothing, and price is not the barrier to instantiating it — integration effort is.

The SDK exists to remove that effort. Every conforming implementation makes the model harder to displace and the corpus more useful to the next participant.

This is a hypothesis, not a result. Free tools attract evaluation rather than commitment, and current adoption is too small to distinguish the two.