Busybox is the exception. The SFC actively enforces the license for busybox. In addition, once you lose your right to use busybox as a consequence of a license violation, the SFL will let you ship it again only if you come into compliance on for all of the GPL code you ship.
So they are making their own busybox as a way to continue to violate all the non-busybox GPL code they use.
If you comply with the busybox licence, you can continue to violate the licence on all the other GPL code. But violating busybox means you have to comply with all of your GPL code.
You never have to release your non GPL code.