Quite simply there is no requirement.

The only things an airline is actually required to compensate for are the few things that have grabbed legislators' attention, such as overbooking (which followed a campaign against this practice).

They are certainly required to refund any money paid if they do not transport you (and you have complied with their other terms and conditions), but that is it.

From a commercial perspective, of course, they may voluntarily wish to do so, to maintain your future custom and goodwill or avoid the story getting into the press. But that is their choice. You may also try to get them to arrange a "last minute" booking on another airline without paying the walk-up fare, which they certainly will be in a better position to arrange than you are.

It is also worth looking at the detailed terms and conditions on your ticket because while these are normally restrictive there may be something in there they have contracted to do that they have not yet told you about, like a commitment to arrange alternative transport for you.