According to the GPL licence, which is the one used by the Linux kernel, every modification in the source must be open source too. But, for example, if I create a new OS based on Linux, I develop my own Desktop system and all that stuff, but I don't make any modification to the kernel, does my project need to be open source just by using Linux?
Here's the preamble to COPYING, included with the kernel source:
“NOTE! This copyright does not cover user programs that use kernel services by normal system calls - this is merely considered normal use of the kernel, and does not fall under the heading of "derived work". [...] note that the only valid version of the GPL as far as the kernel is concerned is this particular version of the license (ie v2, not v2.2 or v3.x or whatever), unless explicitly otherwise stated.
So, if you want to create your own operating system userland from the ground up, then you can license that part however you like. You can then distribute the whole thing together, and the kernel will be licensed as the kernel is and your userland pieces licensed the way they are. This is not uncommon, since various proprietary systems use the linux kernel (although they would often include other open source pieces too, I think).
What you cannot do is distribute the whole thing together claiming your license applies to the included kernel (unless your license is GPL compatible).
You can license your work as you see fit as long as you respect the Linux Kernel Licence.
And you have the right to license your work as you see fit as long as you respect the Linux Kernel Licence.
So, by accusing any Linux distro of being Closed Source just because the Developers choose to Copyright their Original Work, it's slander at best.
And the same goes for accusing Developers of stealing another distro, just because they choose to Copyright their Original Work.
Can a Linux distro be partly closed source (like 95% open source or something)?
Sure. Many are (as many include things like firmware for certain devices, and that firmware is often closed source - but not just, for example previously many included Adobe Flash, which again is closed source, or various codecs for playing back multimedia content).
Free software does not mean lack of copyright. Anyone can distribute their source code freely and they can put any restrictions they like on the source code or on the distribution of binaries they built themselves.