It is possible that the creators of this app have obtained permission from the various news sources to do this. If so it is perfectly legal. If not it would be copyright infringement. (It is hard to see how this sort of thing could possibly qualify as fair use or fair dealing.)
But copyright infringement is only stopped when the copyright holder takes action, by sending a takedown notice or filing an infringement suit. In the US the holder must complete registration before filing suit, unless the work had been registered already (or unless the author was not a US national). So action may be in progress.
Note that simply directing a browser to a copy of an article posted by, or with the permission of, the copyright holder, is a different thing, and is not infringement, although both cause the text of the article to appear on a computer screen. When a browser displays a posted page, it is not thought of as making a copy of the page (although a temporary copy is made within the local cache). But republishing an article is exactly making a new copy, and so is not permitted without permission. The right to control the making of new copies, and grant or refuse permission for doing so, is the central right of copyright.
Also, note that a publisher can, if it chooses, take technical measures to prevent a viewer from downloading a copy of a displayed page. If a reader downloads copyrighted content, the reader may be committing infringement, the browser maker is not. If a reader downloads and then distributes copyrighted content, s/he is almost surely commuting infringement, but suit is unlikely unless this is done on a widespread basis.
As a practical matter, viewing the publisher's site means viewing the publisher's advertisements, viewing the publisher's links to its other content, letting the publisher choose whether to require a login (free or paid), and in general giving the publisher control. A mirror site does none of this. If it makes money, it goes to the mirror, not the publisher.