The Android kernel which is present in EVERY single ROM is an ever-growing community project. Android devs from all around the world have put their hard work into an open source project simply in an attempt to make the Android project better as a whole. While I am sure that all of these Android devs made these contributions to the kernels as a result of things that they wanted or needed to see develop within Android, they made their work available to ALL of us so that we could ALL benefit from their talents.
That being said, the Linux kernel is released under what is called the GNU GPL. (General Public License)
Basically this license allows for the Android kernel to be freely used and modified under the condition that the source of the kernel be released to the public. This "release" can be handled two ways, by either providing a direct link to the source where it can be downloaded, modified and reused under the GNU GPL, or by a written statement that offers the to provide the kernel source to anyone whom requests it. While I realize that I have only outlined a small portion of the GNU GPL, the bottom line here is that the COMPLETE and WORKING kernel source that was MODIFIED by anyone MUST be provided to the community, for any version of publicly released software.
You can find the full text of the GNU GPL licence
here:
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