Suppose Alice makes some proprietary software. The source is available, but under a proprietary license which prohibits derivative works and sublicensing. Bob writes some GPLv3 code. Carol prepares a derivative work which incorporates both Alice and Bob's software and purports to release it under the GPLv3, in violation of Alice's copyright. Carol provides source code and otherwise complies with standard GPL formalities. Alice decides not to sue Carol, but threatens to sue anyone who makes derivative works of Carol's software.
Has Carol violated the GPL, and if so, how?
If Carol has not violated the GPL, why don't companies do things like this all the time to subvert the GPL?