When some corporation sells you some proprietary software, the consumer has the FREEDOM to choose if he wants to fork over his cash in exchange for it.
When the proprietary developer encounters some piece of GPL software, he's being forced to comply with it, even if it's still his own decision whether or not to incorporate the code in his project.
Ah, language.
(TL;DR: nobody is forcing anyone to accept the GPL. The GPL offers you a deal - passing forward the four freedoms is just the price of using the library.)
When the proprietary developer encounters some piece of GPL software, he's being forced to comply with it, even if it's still his own decision whether or not to incorporate the code in his project.
Ah, language.
(TL;DR: nobody is forcing anyone to accept the GPL. The GPL offers you a deal - passing forward the four freedoms is just the price of using the library.)