Seems to be a problem with their online shop. If you open up one of those ePub files in a text editor you'll online find an error message (since their server generates the ePub file every time you click on the download link instead of just providing a previously uploaded file - and that process seems to be faulty). So, nothing you can do until they fix it.
I'm not sure though if this problem is only related to those free books or all books that lack a list of available formats (several non-free books do too). I recently bought 시간을 달리는 소녀 (which lists ePub as available) from them and it works great.
