What's the rationale behind Fillable PDF's not being able to be saved?

Most of the time, if you open a fillable pdf on Windows w/ Adobe Reader, it warns you that you cannot save the data you entered. However, when opening the same file with Preview on Mac OS you are able to save. This seems to be a choice. Can someone enlighten me?

Short version: it's some Adobe DRM of some sort. Apparently you have to have the "right" to save the form.

I always just use CUPS to print to PDF.

Shamelessly stolen from Adobe forums:

Saving a filled out form to the hard drive or emailing it, from a PDF displayed in Reader requires Reader Extensions. It doesn't matter what Acrobat version you are working with. This has always been true.

You can apply Reader Extensions (also called Rights) to a PDF with Acrobat 8 professional. Look on the Advanced menu item.