Some organizations start offering life memberships or subscriptions when they are in need of an immediate cash infusion.  Other more prudent outfits (maybe ARRL) start doing so when their calculations show that the money they receive for the life membership up-front will earn them more than the cost of servicing the individuals' membership/subscription over the long-term.  

For example, investing the life membership money for John Smith should produce more income than it would cost to support that member for XX years.  No need to be "losing money" unless your cost growth expectations turn out to be out of whack relative to investment growth.

Don't forget that the members/subscribers take a risk too.  Think of all the people who bought a lifetime subscription to 73 Magazine, for example, and then it went entirely out of business.  I don't know if Ham Radio or CQ Magazine offered lifetime subscriptions but they're both gone too with no recourse.