The dependency on spidermonkey (which pulls in also python-2.7) is redundant. The spidermonkey plugin is not built by default anymore and the ebuild is not asking for it. Consult the source file avidemux_plugins/ADM_scriptEngines/CMakeLists.txt where it is commented out. Cf. also Bug #721710.
The ebuild could use some polishing and more use flags to make more plugins optional and use system libraries, e.g. the python scripting plugin, ffmpeg dependency etc.