Contrary to popular belief, the Linux kernel from kernel.org is not free software. It not only recommends non-free software but also actually contains it (i.e., software that does not respect your essential freedoms). Because of this, the default install contains non-free software. Here are some links with more info and lists of the non-free software in the kernel:
The mainline kernel does not meet the stricter Free System Distribution Guidelines, nor the Debian Free Software Guidelines, or even the promise made in Ubuntu's Philosophy. Even if by default, some non-free software is included or recommended, there should, at the very least, be a reasonably easy way to choose to have a fully free installation. Modifying the existing kernel is another option-- whichever makes more sense as long as a fully free install is, at the very least, possible. Ubuntu even has a free software only option available...which still contains binary blobs!
Non-free software has been in the mainline kernel for some time, so until (and if) that is fixed, the Linux-libre kernel has already done the work of maintaining of fully free kernel which is used in a number of free GNU/Linux distros. Linux-libre is a project to maintain and publish 100% Free distributions of Linux, suitable for use in Free System Distributions, removing software that is included without source code, with obfuscated or obscured source code, under non-Free Software licenses, that do not permit you to change the software so that it does what you wish, and that induces or requires you to install additional pieces of non-free software. Ideally, the default kernel should be free, but otherwise a fully free kernel should be supported.