I'll guess it's because speed is less of a concern when programming in Ruby.
if you don't want to return any value, then just "return".
Yes, but that leads to clutter and goes against one of the selling points of CoffeeScript.
Previous discussions on this:
http://github.com/jashkenas/coffee-script/issues/1401
http://github.com/jashkenas/coffee-script/issues/899