Is it right for a conference to demand proof of purchase of flight tickets? After I have been accepted to present a paper at a conference, the organisers are asking me to fill in a registration form (participation and attendance are free) AND to produce proof of purchase of flight tickets. I find this bizarre and intrusive. Anyway, my travelling would happen between two capital cities in Europe (plenty of flights) and they demand the proof of purchase to include people in the first draft of the programme. It is a 3-day conference, so I would have to make travel and accommodation arrangements for 5 days many months in advance.
I do not think that the conference organizers have made good decisions about how to handle things.  However, it is understandable that they should want firm confirmation that people are actually going to show up.  Since they are operating a conference with no fees attached, it is unfortunately too easy for people to sign up, hoping to attend, but then cancel when it turns out the the time or funding are not available.  This is a regular issued for conferences and meetings without any direct fees attached, and there are several ways to deal with it.

Some conferences rely on personal relationships with the people who will be participating and presenting.  If the people who are going to be giving talks are known acquaintances of the organizers, these connections can be leveraged to ensure that the registered participants will indeed show up.  (The day after tomorrow, I am leaving for a free conference, which I am attending even though it conflicts with a family event that I might like to attend.  I know the people running the conference, and if I bailed out, it would cost me in terms of my professional relationships.  Since I committed to attend many months ago, I am sticking to my commitment.)

The people running this conference are taking a different approach.  They are not willing to put people on the presentation schedule unless they can verify that they have made travel plans.  While I can understand the motivation for this decision (they do not want people who are slated to give talks to cancel, leaving gaps in the program), I do not think that it is a good approach to take.  Asking for verification of flight information seems intrusive and unprofessional.  I see no reason to believe that the organizers are acting in bad faith, but I suspect that they may not be very experienced in running academic conferences.  So whether you feel like complying with their request for flight information depends on how you feel personally about this unorthodox request.