Viral Licensing
Copyleft licenses are sometimes referred to as "viral licenses" because any works derived from a copyleft work must themselves be copyleft when distributed (and thus they exhibit a viral phenomenon). The term 'General Public Virus', or 'GNU Public Virus' (GPV), has a long history on the Internet, dating back to shortly after the GPL was first conceived. Many BSD License advocates used the term derisively in regards to the GPL's tendency to absorb BSD licensed code without allowing the original BSD work to benefit from it, while at the same time promoting itself as "freer" than other licenses. Microsoft vice-president Craig Mundie remarked "This viral aspect of the GPL poses a threat to the intellectual property of any organization making use of it." In another context, Steve Ballmer declared that code released under GPL is useless to the commercial sector (since it can only be used if the resulting surrounding code becomes GPL), describing it thus as "a cancer that attaches itself in an intellectual property sense to everything it touches". The term 'viral' may be read as an analogy of computer viruses. According to FSF compliance engineer David Turner, it creates a misunderstanding and a fear of using copylefted free software. David McGowan has written that there is no reason to believe the GPL could force proprietary software to become free software, but could "try to enjoin the firm from distributing commercially a program that combined with the GPL’d code to form a derivative work, and to recover damages for infringement." If the firm "actually copied code from a GPL’d program, such a suit would be a perfectly ordinary assertion of copyright, which most private firms would defend if the shoe were on the other foot."
Popular copyleft licenses, such as the GPL, have a clause allowing components to interact with non-copyleft components as long as the communication is abstract, such as executing a command-line tool with a set of switches or interacting with a Web server. As a consequence, even if one module of an otherwise non-copyleft product is placed under the GPL, it may still be legal for other components to communicate with it normally. This allowed communication may or may not include reusing libraries or routines via dynamic linking — some commentators say it does, the FSF asserts it does not and explicitly adds an exception allowing it in the license for the GNU Classpath re-implementation of the Java library.
One should also note that on one hand this 'viral' effect is a normal property of any conventional license on derived works of non-copyleft free material, and on the other hand it is the intended effect when using BSD-licensed works as part of proprietary software. The GNU project using BSD code is in this respect no different from Microsoft or Apple using BSD code, despite claims by proponents of the GPL that it is a freer license than the BSD License.
Read more about this topic: Copyleft