I assume for whatever reason tour group was the best option for IRROPs. Had they only bumped half the tour group there would have been a thread about that. It sounds like (though we don't have all the facts) there was an oversell and the tour group was the best candidate, they moved the whole group (to still keep them together) to the later flight that is still on DL metal, and gave them 24 hours notice (and now they get 4 extra hours in Greece).
In the grand scheme of things that's about the best case scenario you can hope for -- they are lucky that there are two ATH - JFK flights, many other secondary European cities only get 1 daily flight (though some do also get a 2nd for seasonal) and an IRROPs situation would mean either connecting through Europe or waiting another day (and possibly more depending on loads)