The 1968 "Carterfone" case permitted third-party equipment on to AT&T's network. To that point, restrictions on hooking up unapproved equipment was based in safety. With copper wires you literally could kill someone or burn down an exchange with misconfigured gear. Still, the bill passed on the grounds of fair commerce. With fibre, none of that can be claimed. At worse a misconfigured fibre transceiver might affect other communications and would be traceable quickly, as with coax.