Software Freedom Conservancy does enforce copyleft licenses, including suing violators. Their current lawsuit against Vizio is interesting because they aren't suing as copyright holders, but as recipients of GPL binaries from Vizio, they say that they are a third-party beneficiary of the GPL and thus they ask the court to force Vizio to release the relevant source code. Thus they create the possibility for any user of GPL binaries to sue for source code release.

https://sfconservancy.org/copyleft-compliance/vizio.html