So what's the difference between all the books that are sold on ebay, and software?
I think the reason they do this is because, although the CD will have security protection in it, there will be someone there ready to break it and copy the CDrom (the old saying, for every person there is devising new security methods there are 1,000 waiting in the wings to try and hack it).
Once the security code is broken it would be quite easy to burn a load of copies in jig time. With the book of say 500-1000 pages, standing over a photocopier reprinting each page would not be so easy. And if you are prepared to buy a 2nd hand copy rather than a legit copy, you are more likely to try and copy it for your mates or even, dare I say it, for profit.
Just my tuppence worth, but I do agree with a previous poster in that a lot of time and effort goes into making these CD's and quite rightly the owners should protect their property, in the future it may lead to more and improved computer based learning packages being produced.