In this situation it doesn't matter that yahoo is a private corporation - the same cost/benefit analysis essentially needs to be done no matter what the structure of the organization. Let's pretend that email had been created by a government agency and that agency has to decide how much of the budget to spend on security. If it costs X dollars to make something 90% secure, 10X for 95% secure and 10,000X for 99.9999% secure, etc etc eventually you have to choose how much to spend - resources aren't infinite for that government agency either. (And to make it much more difficult, they just have a guess that X dollars will make their product N% secure.) It isn't as black and white as you are trying to portray it.
I think it is fair to criticize yahoo for how the prioritized security but the same kind of issue has happened with non-profit companies and with government organizations, so no, it isn't just a "big, bad, greedy corporate problem."