In a situation like that, if they told the truth, there would probably be a riot at the gate.

Accommodating passengers due to a flight cancellation, unless it's a force majeure event (in other words out of the airline's control) is treated the same regardless of whether it's for aircraft substitution purposes or an actual mechanical. Rule 240 applies.

A summary of what you are/are not entitled to can be found at http://www.onetravel.com/rules/rules.cfm