If they have paid to register for your conference, it’s your duty to write those letters.
I doubt people having speaking slots at the conference will just skip out to go vacation. It’s not like they can’t vacation after the talk. More likely that they just didn’t get the visa because European visa processes, while better than US ones, are just slow.
I doubt people having speaking slots at the conference will just skip out to go vacation. It’s not like they can’t vacation after the talk. More likely that they just didn’t get the visa because European visa processes, while better than US ones, are just slow.