PANIC...someone trying to make money out of free software...
This is a company that tries to make money supporting their distro. To do so it HAS TO attach its own "stamp"...its trademark. This is often incorporated within the distributed binaries. It also has to protect itself from liability from issues arising when someone else uses their derivative works.
The distro, by the rules of open source, is distributed with all the sources available.
Anyone can take advantage of the packaging, configuration etc that Ubuntu has done through developers it has paid, and thereby create their own distro, based on Ubuntu...but for Canonical to make any money out of this, it HAS TO ask that those that repackage Ubuntu into their flavours to remove that "stamp" that Ubuntu has applied. This constitutes a licensing agreement that is not contained within GPL and has to be by definition added later.
Nowhere does it remove rights of redistribution of the sources. Let's not forget that without Canonical, Linux would not have even the 1.5% penetration Linux has amongst the desktop user...