am I correct in thinking the main point of this is just to avoid ticking people off to the point where they would WANT to make a legal challenge to the ongoing copyright violation?
Pretty much. The rules changes are part of an effort to lend us a gloss of legitimacy and to demonstrate that we at least believe we're acting in good faith. The flexibility with regard to older material has more to do with the likelihood of it sparking a challenge. Of course, there are some older comics that remain extremely important to copyright holders. Anything that's part of an ongoing franchise could set off the Big Two.