Furloughs In Ohio

From the Columbus Dispatch:

Some employees of the Ohio Rehabilitation Services Commission will be forced to take unpaid furloughs for no good reason ...

The 550 employees of the Bureau of Disability Determination are paid out of federal Social Security Administration funds but work for the state. The bureau determines whether disabled Ohioans are eligible for federal Social Security benefits. ...

So why is Ohio making the employees take the unpaid time off?

It's required under a union contract covering thousands of state workers, said Steve Ostrander, spokesman for the Ohio Rehabilitation Services Commission. The 550 Bureau of Disability Determination employees are covered by an Ohio Civil Service Employees Association contract that requires workers to take 20 unpaid days off over the next two years.

Why would the union object to the Bureau of Disability Determination employees being exempted from the furlough? Has this been raised with the union?