Microsoft ASF/WMA also has a similar limitation, where it disallows writing an open source implementation if you use the official format specification.
Microsoft ironically made its fortune by selling software that ran on IBM PC compatible x86 boxes, built around PC BIOS's that started from a clean room reimplementation of the reverse engineered IBM BIOS. IBM itself was not very significant in that market except at the very beginning. Clones for me, but not for thee.
Doesn't mean it would in court.