So that everything gets mapped with just one further push of a button.
Well, so the UI convention that users generally expect from a list of items is that all items are unchecked by default, with options to select all and select none. This is what we have right now. So to map all search results, we just need two clicks (click the "all" link, then click the "map selected layers" button).
The explicit "click all" is useful, since there might be several dozen results, and adding them all accidentally would cause a very slow UI experience as all of those layers were getting rendered.