FDK-AAC's license is regarded in the FOSS community as open-source, but patent encumbered. So it's up to the party shipping to the codec to determine where they fall on the spectrum of shipping patent encumbered software. In Fedora, for example, a modified version of fdk-aac is shipped, stripping out the features and functionality restricted by patents, in the fdk-aac-free package, with the more complete package shipped in RPM Fusion.