Funguy,
I suppose that as with many airline policies, it may be harder in print than in practice. According to policies at US, "Changes or cancellation must be made prior to departure date or the ticket has no further value" and at AA, "the itinerary must be canceled before the ticketed departure time of the first unused coupon, or the ticket has no value." These are fairly recent (2002-ish) policies so perhaps your knowledge is from prior to that? Or, as I mentioned, perhaps these are unenforced rules. Technically, though, that is the policy at legacies as well.
Now...if these policies are ignored at the legacies, then there is another important difference between LCCs and legacies...consistency. The LCCs have policies that they enforce and there is little grey area. Legacies, on the other hand, tend to manipulate the rules to fit the mood of the day and that is what upsets customers. They can get a different answer to the same question every time they ask because the true policies are not enforced.
So to ramble more...I guess that it isn't that the legacies have to create the same beneficial policies as the LCCs...they just need to ENFORCE them!!