Businesses use limited amounts of debt to fuel growth because they have a justifiable case that shows that the servicing of the debt is less than the returns on the investments made with the debt.
Often, businesses are wrong about whether or not their "justifiable case" is true. When wrong, their debt may just cut into the other assets on their balance sheet. Perhaps creditors will allow them to refinance so that their debt servicing is below their investment. Perhaps they'll go bankrupt. Perhaps they'll close their doors.
The US government 1. Has a really crappy track record of finding a return for its investments. 2. Will cause much more world disruption if it has guessed wrong on the whole "returns on investment vs debt servicing" equation.
Finally, businesses do not borrow money without limits. Their borrowing of money is based upon the amount of credit that outside parties give them. Given the US Government's downgrade in its rated ability to service its debt, shouldn't we be concerned that we're overextending ourselves?
