The availability of the drag and drop feature depends on your browser and on the current runtime. Looking at Plupload sources, as far as I know, the feature is only available on HTML5 runtime (your examples differ on the runtime order, so run on different runtimes, some of which do not feature drag and drop)
To enable drag and drop on modern browsers, you should put html5 first in your runtime list :
runtimes : 'html5,gears,flash,silverlight,browserplus',