They are using DRM. Quote from their site:
Quote:
If you are using Microsoft Reader eBooks…
Microsoft Reader eBooks may be moved from computer to computer, as long as each computer has a copy of Microsoft Reader that is activated to the same Passport account as the computer used to download the ebook.
If you are using Mobipocket eBooks…
You can read Mobipocket eBooks on 3 machines. You need to install Mobipocket Reader on any machine on which you intend to read a Mobipocket eBook. You must then register a Mobipocket PID at this site for each machine on which you intend to read a Mobipocket eBook.
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