Harold Welte has copyright in BusyBox and is the most prolific enforcer of the GPL in the world. The SFLC does Harold's enforcement actions in the US. I should also point out that once SFLC has been involved they ensure compliance with all GPL applications, not just Busybox. The beauty is that Busybox is almost a requirement in the embedded space if you are using any GPL code and make it an easy target for enforcement because it's easy to find, easy to prove and heavily enforced making it a slam dunk case.
In addition, once SFLC files suit posting the code isn't enough, they usually revoke the license and refuse to reissue until they are reimbursed for legal fees + a little extra for the next action and almost always require the companies involved to appoint a GPL compliance officer so it doesn't happen again.