Please do not use the editing API for such downloads, as the name says it is mainly intended for use for editing.
To export a subset of OSM data for a area you should either use an [extract of the area](http://planet.openstreetmap.org/) in question and filter to obtain (please have a look at the [wiki](http://wiki.openstreetmap.org) for descriptions of the many tools available) the data that you want or, if the area in question is not too large, use the [Overpass API](http://wiki.openstreetmap.org/wiki/Overpass_API).