I am not sure that is true given the interpretation of the fair use laws (see http://www.utsystem.edu/ogc/intellectualproperty/c opypol2.htm
) I would think trailers would be less that 10% of the total film and in addition there are inherently a production that the distributer would want to be viewed by as many people as possible.
They may be less than 10% of the whole film, but then again the trailer is a work *in itself*, so if you're distributing 100% of the trailer then you're distributing 100% of a copyright work. It's still illegal.