This is such a bogus argument. You are talking about people who buy their kids iPods and cell phones, and wardrobes that run into the hundreds of dollars. Who live in houses with multiple bathrooms, and think they are really poverty stricken if both their cars are more than 5 years old! Any expressed concern for "the poor" is crocodile tears.
Yes, Charity, we all know there are no truly poor in America, much less in the Mormon church.
You live a very insulated life.
This is what happens when a person comes back to full activity. They don't have to make up back tithing. In essence, it is "forgiven." Now, if they come into the bishop's office and tell him they want to go to the teple next week when their kid gets married, and oh, yes, here is my tithing for this week, that won't really wash. There has to be a demonstration of sincerity. I don't have the mantle of a bishop, nor his discernment, but from my less than perfect point of view a person who decides to pay tithing because he wants to go to the temple for a special occasion would not pass the sniff test.
The blanket forgiveness was immediate, and applied to everyone who had gotten behind in tithing. In other words, he didn't demand a time period to demonstrate sincerity. No, it's not the same thing as an inactive returning to activity.