I had a look at http://www.playwitharena.com/?Legal_Stuff
and I would say that Arena is copy-righted freeware (I am not good at licenses and I do not know whether such a term exists):
You are allowed to download the program package and install it on as many computers as you like free of charge.
However, distribution or offering of Arena Chess GUI, SOS for Arena, Dragon, AnMon, Nejmet, Ruffian, Betsy, Chispa, Hermann, Frenzee, Spike, Aice and ELOstat on other webpages, newspaper, CDs, print and other media is only allowed in unmodified form and with the written permission of their respective authors Martin Blume, Rudolf Huber, Bruno LUCAS, Christian BARRETEAU, Jean-Christophe GABILLARD, Per-Ola Valfridsson, Landon W. Rabern, Federico Andrés Corigliano, Sune Fischer, Volker Annuss, Volker Böhm & Ralf Schäfer, Anastasios Milikas and Dr. Frank Schubert.
I suppose that the developer is careful with the licensing because as I mentioned in my previous posts Arena comes with several external chess engines which might be free but might not be open-source. Arena installer also uses Inno Setup.
About Softpedia- Arena is published in Games section of the site and bear in mind that in Softpedia Games there is no license information about the products.