Then why does Coinbase lock accounts of people to transfer to darknet markets or such?

If you had the choice of receiving BTC from a brand new minted batch, or from a darknet market, which would you prefer? If you have a preference, then they do not have the same value.

I would value BTC from a darknet market less because of risks associated with spending it.

I would value freshly-minted BTC because it doesn't have any question about its providence.