Interoperability has never been legally mandated in the past, but for most of history it has just "happened" because people could get around hostile designs (IBM PC clones anyone?).
The problem with the complexity of today's technology is that, it's no longer feasible for an upstart to make a "compatible" device. You can't engineer an "iOS capable" non Apple device or an Apple-compatible non-Apple OS, not because it is not legal but because it is not feasible technologically. There are ways to do legally clean reverse engineering (ReactOS anyone?) but you cannot get around chip level signature checks nor can you fab your own state-of-the-art chips.