Contracts, exclusivities and embargos in US festivals has nothing to do with European ones. You cannot compare Boston Calling or US festivals industry with European one.
When an European big festival books a headliner, normally they sign in the contract that they cannot announce another show in the same country until the festival happens. You won't have this problems in the States where the festivals share their headliners.
European (and Spanish) contracts with lot of bands have lot embargos and exclusivities. I remember Resurrection Festival announcing a headliner as "only show in Spain in the next two years".
Be patient, wait until Primavera happens and then analize if Mad Cool have taken right decisions keeping silence. Their festival is sold out, as well as Werchter is, so they can act the same way that Werchter and do not replace the big bands that have pulled out or spend money in booking big bands once the festival is sold-out. Let's see...