Why can't I edit attribute table imported through 'text as layer'

By default CSV layers cannot be edited. You have to save to a different format, e.g. Shapefile, before you can start editing.

Update 2015-11:

There's a new plugin that solves this issue called Editable GeoCSV. It can handle x and y columns for points or a WKT column. For more details see http://giswiki.hsr.ch/Editable_GeoCSV_QGIS_Plugin