The default behaviour looks for @license or @preserve...
But many plugins and libraries are still using /*! for licensing comment blocks...
How can I use UglifyJS2 to preserve comments starting with /*! ?
See https://github.com/mishoo/UglifyJS2#usage
Using the --comments argument, you can supply a regular expression.
uglifyjs jquery.plugin.js --comments '/^\/*!/' -o outfile.js