Why does add-apt-repository fail to add source repositories?

If you want to add your repo as source repo as well you have to put the -s flag in the command line:

sudo add-apt-repository -s 'deb http://dl.ajaxplorer.info/repos/apt squeeze main'

And thats all.

This will add:

deb http://dl.ajax...

and

deb-src http://dl.ajax...

as well.

The man page for apt-add-repository says (emphasis added):

REPOSITORY can be either a line that can be added directly to sources.list(5), or ...

In the first form, REPOSITORY will just be appended to /etc/apt/sources.list.

Now, while deb-src is a legal type for sources.list and you may wish to file a bug, it appears that this will do the job just as well, since GPG keys are only downloaded for PPAs:

sudo sh -c "echo deb-src http://dl.ajaxplorer.info/repos/apt squeeze main \ >> /etc/apt/sources.list"