Based on Wikipedia Ubuntu is covered mainly under GNU GPL. In my company anything related to GPL means - can't be used, they are very afraid of legal traps related to GPL.
That's why I have this question: can Ubuntu Server on Amazon EC2 be used as a host OS for commercial web application?
If yes, is there any legal documents proving this? I found About Ubuntu Licensing. Is it enough to be sure my company is safe from any legal issues?
Absolutely you can do this. Many companies all over the world run a portion (sometimes all) of their commercial infrastructure on Linux distributions. Ubuntu in this sense is no different from Red Hat, Debian, SUSE, and the rest.
Just because the kernel and many system components are under the GPL, that in no way means the code you run on top of it must also be free or open source software. A simple search for commercial software and commercial web services that run on Linux will prove this quickly.
One simple example would be Google. Virtually their entire server infrastructure runs on Linux, but clearly that hasn't forced them to give out the code to their web indexing software, or Gmail, or G+ (not to mention the entire Android platform which runs on Linux).
As for being sure enough to convince your legal department that it is ok to run Ubuntu Server, you might consider buying an Ubuntu Advantage support contract from Canonical. They (ok, we) include legal indemnification with each support contract. Please see:
http://www.canonical.com/enterprise-services/ubuntu-advantage/assurance-terms
http://www.canonical.com/enterprise-services/ubuntu-advantage/overview
Also: I didn't mention EC2 specifically because there is no reason to think that hosting it on EC2 would be any different compared with running it "in house" from a legal standpoint.
Also, too: In the case of Android, let me clarify. Android's modifications to the Linux kernel are GPL'd, however the rest of the platform is on a very permissive Apache-style license, and obviously a great many companies sell paid, closed-source software that runs on it.
If you need to be really sure you need to contact a good lawyer who knows about the laws in your country.
I'm nor a lawyer and I don't know about your country, but that's what I think:
On http://www.ubuntu.com/ubuntu/why-is-it-free Canonical writes that Ubuntu is Open Source, and on http://www.ubuntu.com/project/open-source they link to http://www.opensource.org/docs/definition.php for an explanation on what Open Source is. Point 6 in this list says:
The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research.
So I'd say that Canonical thinks you are allowed to use Ubuntu as host OS.
Do not know if it helps but you could take a look also to Ubuntu on EC2
