This is well-documented in several places by Magix. Here is the info from page 69 of the manual.
"Note: During AVCHD import, it is necessary to activate (view page 344) the MPEG-
4 codec and Surround Sound Stereo/5.1 for copy protection reasons. MAGIX Video
Pro X6 offers this option as soon as the codec is required."
See also page 345 in the manual:
"Activate codecs
You may activate encoders/decoders for various file formats as required. The
corresponding activation dialog will appear automatically when the codec is used for
the first time, e.g. when a DVD is burned and the MPEG-2 codec is used.
Why does it have to be "activated"?"
The screen that you showed says to download the free CODECs. I presume that you did.