On a normal shared plan you can put anyone you want on it but if you do and they fail to pay then you are responsible for the whole bill, so only a limited number of people would risk this.
But for FRAMILY you could have anyone on your plan to help reduce costs and not be responsible to pay the whole bill. You could put complete strangers on your plan and never had to worry. So many people were doing this that Sprint was losing more money then they had imagined. So they said "it was too confusing for people"
Even the sales people at the Sprint stores themselves were putting complete starngers together to help you save.
In a nutshell it worked TOO well and Sprint lost money.
But of course company execs would never be dishonest, right?