Originally Posted by CO_Nonrev_elite
It is actually illegal for the airlines to cancel flights for these reasons. As mentioned above, sometimes carriers will develop a "mechanical" and cancel, but in general they just fly them with the low load
That's not true. They are required to re-accomodate you but are allowed to cancel flights for pretty much any reason.
Often, flights with low loads are cancelled to re-balance a schedule after a mechanical cancellation and/or weather related cancellations. That way, the airline can get back on schedule without flying empty planes around.