What are the reasons for disabling the saving of data in a PDF?

Question

I seem to have had more than my share of encounters lately with PDF files that have editable fields but have their data saving capability disabled.
In some of these situations this has been an absolutely maddening inconvenience.
When I attempt to bring this to the attention of whomever's responsible I get brushed off.
One person's response was: Actually the majority of the PDF forms I have are Adobe reader and you cannot make changes electronically at all.  The reason for this is to make sure that the documents are not altered in any way.  The <companies creating the PDFs> must make sure that changes are not being made to their forms. 
I just don't understand that.  How is entering and saving data 'altering' a PDF?
Is this just bone-headed bureacracy, an urban legend, or is there some fundamental practical matter I am not aware of?

Answer 1

Saving a filled-in form has always been disallowed with Adobe Reader, formerly known as Acrobat Reader. Though there are a few exceptions...
History lesson:
Forms features were first introduced in Acrobat 3. As you might imagine, a number of folks wanted to be able to save filled-in forms using Acrobat Reader. Since this functionality simply was not a feature of Reader, it was not possible. If you wanted to save a filled-in form, you needed to use Acrobat (Exchange).
Upon recognizing the potential market for a product that they could sell, Adobe developed and released Acrobat Business Tools, which was a version 4 product. It was positioned between Reader and Acrobat, was reasonably priced, and allowed users to save filled-in forms, apply digital signatures, and add comments. Unfortunately, this was marketed poorly and did not achieve commercial success.
The next iteration was a similar product named Acrobat Approval, which was a version 5 product. It too could save a filled-in form, but was in some ways more limited than Business Tools, and suffered a similar fate. That is, it was not a market success. It was however distributed for free to US taxpayers on the IRS tax products CD-ROM.
The next idea from Adobe - which was a good one - was to provide for a means of embedding usage rights in a PDF document. These rights would allow Reader to do things it normally would not be able to do, such as saving a filled-in form, adding comments, importing/exporting form data, creating new pages using templates, connecting to databases and web services, etc. At first, the only way to enable these usage rights was with an expensive product from Adobe, so the potential was initially not realized.
With the release of Acrobat 7, Acrobat users were able to add some usage rights, such as commenting. While this was a welcome addition, folks still wanted to be enable forms to be saved with Reader. Finally with the release of Acrobat 8 Pro, users were able to enable forms to be saved with Reader, but it came with licensing restrictions. In short, if the licensee distributed the form for more than 500 recipients, the licensee could use data from no more that 500 copies of a particular form that was returned to them, including hardcopies. This was great for forms used inside of an orginization, but not so great for forms that needed to be used by the general public. Still, it was a very useful feature for a majority of businesses for internal forms, the vast majority of which have fewer than 500 employees.
Acrobat 9 or 10 didn't change things. Adobe still wants to capitalize on this market, which certainly is not a bad thing for Adobe and their stockholders. The US IRS forms have long been saveable with Reader because they licence Adobe Reader Extensions, which provides revenue for Adobe.
Note that some third-party PDF viewers allow users to save filled-in forms whether or not the documents have usage rights. Such software has a different marketing model than does Adobe Reader/Acrobat, and rides the coattails of Adobe by taking advantage of the popularity of PDF forms.