Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.
That's from section 0 of the GPL. Every person who has ever written that "the GPL restricts the use of GPLed software," or that the GPL is comparable to an end-user license agreement (EULA), thereby evidences having not read, or not understood, the GPL.
(This is not a defense of the GPL against critics who favor the BSD-style licenses on grounds that they are "more free" than the GPL. That is another argument for another time. It is specifically a defense of the GPL against those who consider it morally or legally equivalent to a Microsoft-style EULA.)
License to Use, with Copying Forbidden vs. License to Copy, with Usage Unrestricted
Take another look: "The act of running the Program is not restricted." For nonprogrammers, this is the chief practical distinction between the GPL and an EULA. An EULA asserts control over whether and how you may run the covered software, whereas the GPL explicitly denies such control. An EULA asserts that the copyright owner may revoke your right to use software you've legally obtained, whereas the GPL recognizes that copyright only affords the owner the right to control your copying of the software.
If you are a programmer or distributor, of course, the distinctions between the GPL and an EULA are much more distinct. EULAs such as Microsoft's don't just exercise copyright, i.e. forbid you from making copies. They typically order you to refrain from analyzing or learning about the covered software -- for instance, disassembling or tracing it -- even though these are user rights that copyright does not restrict. That is to say, EULAs claim to leverage copyright to gain control over actions which copyright doesn't itself cover. In contrast, the GPL permits you to do things which copyright normally disallows, viz. making copies and derivative works, albeit only under certain terms.
Terminable Subscriptions vs. Interminable Permission
As proprietary-software houses move increasingly towards subscription-based software licensure, we may see a new distinction for end users. Subscription software relies on the idea that I can sell you a piece of software with a time limit on your right to use it. Thus, the software may become unavailable for your use for any of several reasons, even though you have a copy in your possession. Since GPL provides no restrictions upon use of software -- only upon copying -- none of these can apply to its covered software:
- Publisher withdraws software from licensure to promote another, more profitable product;
- Publisher is forced to withdraw software due to lawsuit or regulation;
- Publisher tactically withdraws software from your particular market in order to harm a specific competitor;
- Publisher issues license renewals only under restrictive terms which exclude your uses -- for instance, by forbidding use in ways which compete with publisher in new markets;
- Publisher goes out of business, and thereby becomes unable to issue license renewals.
It is fully possible that the author of a piece of GPLed software could be restricted from distributing it by a lawsuit or regulation. It is not nearly so likely that such restriction would deprive existing users of the right to use the software. Even the authoritarian DMCA does not forbid one from using a piece of infringing software -- only from distributing it.
Naturally, many programs both proprietary and free are used to store data in particular formats. Word processors and databases come to mind. Under subscription licensing, users end up paying for the privilege of accessing their own creations.
Innovating Freedom vs. Innovating New Kinds of Unfreedom
Aside from subscription-based licensure, EULA-covered software has of recent come to stand for the innovation of whole new categories of unfreedom. As time goes on, we may reasonably expect that this trend will continue.
For instance, a copy of proprietary software may by default be transferred, like any other piece of property: if you purchase one copy of a proprietary program, you may install it on your computer or give it to your mother to install on hers, but not both. If you want to give it to your mother, you have to erase it from your own system first. Older proprietary copyright notices were particularly fond of the phrase "This software is like a book," meaning that one purchased copy could only be used in one instance at a time. EULA-covered software, however, has come up with new restrictions to forbid this and other acts to which software users are accustomed.
It is the avowed purpose of the GPL to innovate a new category of freedom: a software commons to which all may contribute, but from which nobody may restrict others, with enforcement provided solely by the terms of copyright law. It is the self-evident purpose of EULAs to innovate new categories of restriction: to go beyond the statutory restrictions of copyright, engineering whatever restrictions upon users may be maximally profitable.
One may fairly argue that the GPL does not provide for maximal freedom for programmers, as do the BSD critics mentioned above. However, this is a far and faint cry from the parroted claim that the GPL is morally, legally, or indeed practically equivalent to an EULA.