Why does Silva distinguish free labor from wage labor ?

This is a very good conceptual question.

There are actually two different relationships being discussed.

First: free labor vs. slave labor

Silva is discussing labor regimes.

Free labor was historically constructed as an alternative to slave labor.

Second: free labor is not the merely wage labor

Silva's point is that “free labor” was a broader ideological and legal concept than simply “wage labor.”

A wage worker could theoretically receive wages while still facing coercion or restrictions.

So nineteenth-century thinkers could debate:

What makes labor genuinely “free”?

That is why Silva doesn't simply say:

free labor is the wage labor.

She is examining the historical meaning of the concept, not giving you a dictionary definition.