It is all about the control and extracting more money from the reader. Preventing piracy is just an excuse.
1.
You don't own the content any more.
A paper copy or a PDF copy was ownership. Now you are just "renting" the content that you have no control off and that can be switched off at any time.
2. They are selling PDFs separately on CDs (see Ken's comment above). Let me guess, even though you have digital subscription now you have to pay again for the CDs to get the PDFs.
Basically now they are double-charging for the same information.
Control and money.
Finally, if you google "Zinio PDF" you may get surprised
