They make it a condition of licensing you the ability to encode or decode the video — contract law.
If you read the EULAs and such on even a piece of 'professional' software like Final Cut Studio or Avid, it'll describe how the encoder is only licensed for personal use and that someone else in your distribution chain needs a different contractual arrangement with them, along with whoever's on the playback end.
If you read the EULAs and such on even a piece of 'professional' software like Final Cut Studio or Avid, it'll describe how the encoder is only licensed for personal use and that someone else in your distribution chain needs a different contractual arrangement with them, along with whoever's on the playback end.