embedded linux kernel distribution

I am doing embedded system development with my EVM board. And now, i want to release my whole software to others. To get some referrences on how to package my software, i downloaded some other vendors' linux SDK.
The thing confused me is if its vendor modified the linux kernel, instead of releasing the SDK with a dirty kernel directly they all have a original clean kernel along with a patch containing this modification.
So,that is why? To avoid license? Linux kernel license requires the kernel not be spreaded with personal modification?
Thanks