"As far as I know, this is not an accurate description of this situation."
Of course you uploaded the demo during a holiday, which means you would have been unable to get an accurate description of the situation. Did you contact Electronic Arts or 2015 to ask them whether it was acceptible to post the demo?
"The demo is not leaked, and is not "meant only to be purchased," it's a cover-mount demo that's been uploaded."
...which is only available if you purchase the magazine in question, correct? Is it leaked? No. Is it being distributed without the magazine? It would appear that way, which is probably a violation of the distribution agreement between Electronic Arts and the magazine.
I've seen the legal documents magazines have to sign to distribute a demo; they specifically forbid alternate distribution methods.
"It happens to every single demo ever released, and the companies that distribute demos in this way fully expect it to happen."
Do you have signed permission from Electronic Arts to distribute the demo? I bet the magazines do. If you do not, you are in violation of the terms of their distribution agreement, as are all of the other sites that post it.
And the "it happens to every other demo" is a weak argument. Warez happens to every game.
"the fact is that if any one of the three had actually asked us to remove the demo, we would have done so immediately, but none of the three have said a word."
The fact is they shouldn't have to, and the other fact is this all went down during a holiday.
"If this were actually a product only intended for purchase, we never would have dreamt of posting it... I'm frightened that the point here has been this scrambled."
How is the demo available without purchasing something? Has Electronic Arts made it available on their website? Do all of the websites have a piece of paper authorizing them to distribute it?