The internet reader in this case is lower level than the memory playback which appears to be disabled in the case of playing a URL.
The reader would normally buffer the file to be played back to disc so that it can be seeked, etc if it's not a seekable stream.
Enabling memory playback wouldn't change this since memory playback is done at the playback level which is well above the internet reader level. That's why the option to disable buffering to disc is enabled by default on the IdPi, the file is played back while it's being streamed bypassing disk buffering. At this point, memory playback doesn't offer much, it doesn't help the stream be seekable, etc.
That said we are not committed to play from memory for URL's being disabled but at any rate you would still have to leave disk buffering disabled in the options for your IdPi.