I've seen busybox in a lot of software that's not free. One notable example is VMware. It runs on top of esxi as a package to provide command line functions to VMware hosts.
I'm pretty sure (IDK, I don't do development for vmw) that it's running on top of VMware's kernel, and they have binaries that you execute from busybox that interface with the vmkernel to accomplish things.
I don't have all the details and I'm far from an operating system guru/developer/whatever. I think that's permissible under copyleft, since they're not running things that you paid for on top of busybox, but I have no idea. I'm also not a lawyer, but they've been doing it forever, as far as I know.
Does anyone know more about it? I'm just surprised that smaller fish have fried for infringement, but someone like VMware is shipping busybox without reprocussions.
Maybe it's not busybox? Maybe it's something that just looks and acts like busybox? Idk.