They Get It When It's the First Amendment
There's a good reason rights are not subject to simple majority rule. The media get that--when it comes to the First Amendment, where they will defend unpopular, obscene, or even dangerous speech (rightly so in most cases, though whether such speech is always ethical in the circumstances is an entirely different question).
The First Amendment exists to protect speech you don't like. The fact that a simple majority might oppose the Second Amendment as it is currently interpreted is not a sufficient argument for new gun control laws. If 90% of Americans really do support the president's position, he ought to move a new constitutional amendment.
Otherwise, he is just demagoguing--in precisely the ways the Framers anticipated and feared, over two centuries ago.