In the linked entry Stallman primarily explains what is understood as Free Software. The comparison with Open Source merely serves as a device to illustrate the nuances in a more relatable way.
>Another misunderstanding of “open source” is the idea that it means “not using the GNU GPL.” This tends to accompany another misunderstanding that “free software” means “GPL-covered software.” These are both mistaken, since the GNU GPL qualifies as an open source license and most of the open source licenses qualify as free software licenses. There are many free software licenses aside from the GNU GPL.
While free software protects the freedoms of the user only, copyleft software protects the freedoms of the developer as well. Both types of software are ethical (as mentioned in [1]), so it comes down to personal choice of the original author, who can use it as a tool to ensure their goals.
The author includes RMS' opinion, which is true for this particular piece of software, though in many cases it's a warranted complaint. It gets into a political argument usually, but it boils down to this argument: "If I don't require copyleft, then I can't always guarantee that users of my code will get all the freedoms I provided to them."