might be orthogonal to whether it's proprietary or Free Software in theory (although correlated in practice), couldn't a communication software be Open Source and free licensed BUT ask that users only use the software in a manual fashion and never automated?

A harder version of the clause, that only a specific interface be used, might be contrary to Free Software howeer, as it would infringe in the right to modify (the interface), but not the right to study the software.