And yes, this is in the United States: Philadelphia, in fact.
At first blush, such a sentence sounds harsh, the stuff of repressive banana republics. But upon reading the facts of the case, I believe this is a win-win for all involved.
The alternative would be to put the convicted thief (in this case, a 40 year old woman who embezzled from her employer) in a cage where she would cost the taxpayers a lot of money, would be subjected to possible violence and humiliation, and would be unable to work to pay back the money she stole. To order a person to pay back hundreds of thousands of dollars in restitution while the person is incarcerated makes no sense. It is basically a reversion to debtor's prison. Ultimately, the victim would never see a penny.
In such a scenario, the only "winner" is the state. It also adds to the brutish and thuggish American culture that seems to revel in seeing people suffer in a dramatic way rather than seeing to it that wrongs are righted.
Although 21 years of "house arrest" sounds extreme, it is not based on some arbitrary standard, but rather on the time needed to make restitution. If she pays off the debt sooner, her sentence will be over sooner.
Justice (and mercy) will be served, restitution will be made, taxpayers will be spared, and there will be one less incarcerated non-violent American. This is not to say such a sentence would work in every case (and it may not even work in this one), but there really must be better ways to serve justice than to leave the victims with no restitution and to have people who are not in any danger of violence languishing away in overcrowded prisons.