I remember asking Maxmaps during a Squadcast a long while back (~.22), and believe he said that at some point they might consider hosting the old KSP versions downloads, thought it is certainly (and rightfully) not a priority, considering it would really only benefit a small number of players. However, I am one of those players, so I hope at some point they put them up. As for the reason why you can't share versions with people, even if they also have paid for it, really comes down to copyright law (don't take my word for it). Essentially, from what I can tell, if squad is lax about people sharing paid versions, then someone does a very large pirating scheme, which squad (or any sensible company) would want to stop, they will have a much more difficult time in court, because the pirate-er can claim that squad has not enforced their trademark/copyright (or something like that). This could at the very least negatively affect how much is awarded in damages.