atag doesn't force atag

ffmpeg -i MI-351_8b.mov -vn -acodec pcm_s24be -atag in24 in24.mov
ffmpeg version N-32741-g440e984, Copyright (c) 2000-2011 the FFmpeg developers
  built on Dec 27 2011 09:48:51 with gcc 4.2.1 (Apple Inc. build 5666) (dot 3)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/HEAD --enable-shared --enable-gpl --enable-version3 --enable-libfreetype --enable-nonfree --enable-hardcoded-tables --cc=/usr/bin/gcc-4.2 --enable-libx264 --enable-libfaac --enable-libmp3lame --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libxvid
  libavutil      51. 33.100 / 51. 33.100
  libavcodec     53. 48.100 / 53. 48.100
  libavformat    53. 28.100 / 53. 28.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 55.100 /  2. 55.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  5.100 /  0.  5.100
  libpostproc    51.  2.100 / 51.  2.100

Seems stream 0 codec frame rate differs from container frame rate: 30000.00 (30000/1) -> 29.97 (30000/1001)
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'MI-351_8b.mov':
  Metadata:
    major_brand     : qt
    minor_version   : 512
    compatible_brands: qt
    creation_time   : 1970-01-01 00:00:00
    encoder         : Lavf53.14.0
  Duration: 00:00:13.03, start: 0.000000, bitrate: 169865 kb/s
    Stream #0:0(eng): Video: rawvideo (2vuy / 0x79757632), uyvy422, 720x486, 167793 kb/s, SAR 9:10 DAR 4:3, 29.97 fps, 29.97 tbr, 30k tbn, 30k tbc
    Metadata:
      creation_time   : 1970-01-01 00:00:00
      handler_name    :
                        DataHandler
    Stream #0:1(eng): Audio: pcm_s24le (in24 / 0x34326E69), 48000 Hz, 2 channels, s32, 2304 kb/s
    Metadata:
      creation_time   : 1970-01-01 00:00:00
      handler_name    :
                        DataHandler
Output #0, mov, to 'in24.mov':
  Metadata:
    major_brand     : qt
    minor_version   : 512
    compatible_brands: qt
    creation_time   : 1970-01-01 00:00:00
    encoder         : Lavf53.28.100
    Stream #0:0(eng): Audio: pcm_s24be (in24 / 0x34326E69), 48000 Hz, 2 channels, s32, 2304 kb/s
    Metadata:
      creation_time   : 1970-01-01 00:00:00
      handler_name    :
                        DataHandler
Stream mapping:
  Stream #0:1 -> #0:0 (pcm_s24le -> pcm_s24be)
Press [q] to stop, [?] for help
size=    3666kB time=00:00:13.03 bitrate=2304.4kbits/s
video:0kB audio:3665kB global headers:0kB muxing overhead 0.018598%