I have been thinking that the timestamp thing is there to make it harder to just convert some GIS data from shapefiles or some other GIS format to GPX and upload them. Timestamps added by some utility are easy to recognize from the modified GPX file because driving speeds do not make sense. But I don’t know if this is the real reason. Perhaps because of another secret rule JOSM does not convert GPX waypoints to OSM points even it shows them.