Is GPL Legal?
GPL is enforceable as it's essentially a copyright license. The copyright holders of the GPL software can choose to enforce the GPL on the distributed or derivative works of the software. For example, the FSF holds the copyrights on many pieces of the GNU system, such as the GNU Compiler Collection.
Conclusion. The GPL and other copyleft licenses are not bad; I just think they're pointless. They have practical problems, and they are tainted by the behaviour of the FSF, but most importantly, I do not believe they have been an effective contributor to software freedom.
It's time to say goodbye to the GPL - Martin Kleppmann
Is GPL legally enforceable?
A question mark over whether the GNU GPL – the widely used free-software license – is enforceable as a contract may have been resolved by a US federal judge. ... She ruled that the GNU General Public License – the GNU GPL – is an enforceable legal contract even though it is not actually signed.
For now, GNU GPL is an enforceable contract, says US ...
SCO's Chris Sontag argues there are many reasons that the GNU General Public License is bad for the software industry. The General Public License is not good for the software industry for a variety of reasons.
No, the GPL is not good for the software industry | Network World
Software under the GPL may be run for all purposes, including commercial purposes and even as a tool for creating proprietary software, such as when using GPL-licensed compilers. Users or companies who distribute GPL-licensed works (e.g. software), may charge a fee for copies or give them free of charge.
GNU General Public License - Wikipedia