It's called being layed off.
Generally, when a worker is layed off, they are entitled to unemployment benefits, payed out by the state. This all comes back to haunt the employer as an increase it the employer's unemployment insurance premiums.
If you are working for a company that plans to pull one of these stunts and intends not to pay you for that period of time, head right down to the unemployment office and collect your money. You'll feel better with the money in your pocket, and your company will think twice about this sort of behavior in the future.
Another good thing to do during the week you are layed off is to set up some interviews with other employers, which you may be required to do, anyway, in order to collect your unemployment check. You may need to set up the interviews ahead of time, since some employment offices require that you actually go to the interviews before you can collect your check.
All of that said, there are benefits to employers in this kind of shutdown, even if they still need to pay their salaried employees: first, not all employees are salaried, so there is still some money to be saved from payroll, second, if you can shut off all the equipment in the buildings (air conditioning, lights, computers, etc.) then you can save a healthy chunk of change on utility bills, finally, some services the company pays for may be contracted out, and can be suspended during the down time.
With this rational, it might make some sense for the company to either a) add an extra week of paid vacation to everyone's benefits (and require that everyone take that week at the same time) or b) shorten the work week by one day, and shut the offices down over the longer weekend. A company might even be able to barter the extra time off for slightly lower salaries or wages, and some studies have suggested that shorter work weeks lead to increased productivity.