How does a PDF prevent itself from being printed?

I have a PDF for my camera's manual, and Adobe Reader won't let me print it (the print option is grayed out). SumatraPDF also does the same thing (it even says print denied). How does the PDF prevent itself from being printed? It seems that if the program can display it on the screen, then it can also print it. Maybe Adobe Reader respects the PDF not printing, but surely an open source PDF reader wouldn't be so restrictive. So is there something more to this than merely the PDF reader software respecting the PDF's request to not be able to be printed?

The print permission is in the pdf file itself and is mostly used by Adobe products. Some products pay it attention, and some others print it anyway.
It is quite easy to remove.

See the article How To Unlock Adobe PDF Files, where it explains how to use Freeware PDF Unlocker to remove this and other passwords.

Warning : Comment by Chris Betti says

As of Oct 7, 2013, the Freeware PDF Unlocker setup installs unwanted software without confirmation, and fails to install the product itself.

Use older versions (if can be found) or search for another product.

It's part of the settings when you save the PDF. You can also disallow text to be copied. There are software that allow you to bypass the restrictions.

Nope. As you say, if it can be displayed then it can potentially be printed. It requires the cooperation of the reader in order to enact this.

Try Okular. If you are like most people, you need KDE for Windows.

On Debian-based Linux distros:

$ sudo apt-get install okular

or on RPM-based distros:

$ su -c yum install okular

or you can compile it on other distros.

On the Mac, you will have to install KDE for the Mac. This requires compilation and is probably not worth your time. (Why would you use such a limited OS anyway?)

On other OS's, you may need to compile it, if there isn't a package available.

Okular should be able to get around PDF restrictions by default.