This is a false premise... the only way to buy a house for just the back taxes is if the homeowner owes NOTHING on the property to a mortgage company. What you're REALLY doing is buying the right to collect on those taxes in most states. You are allowed to charge interest too. Buying a house for unpaid back taxes does not wipe out the bank's first interest in the property. THEY own the house, you don't. However, if there is no bank involved, then yes, you can repossess the house in its entirety if the owner of the house doesn't pay the taxes due within a specified amout of time. But quite often, for a home with 100% equity, the homeowner can just get an equity loan to pay the taxes off before the house becomes yours.
That whole TV ad is a SCAM for you to BUY their program. If it was so easy, they'd be doing it themselves all day long and not selling tapes or CD's. Don't fall into that trap.