Note that it's all very round about. It's not that they're cutting employee hours because they can't afford to pay them $15/hr. They are cutting employee hours because they are shifting towards a higher ratio of part time employees -vs- full time employees which takes employees off their medical plan. They are hiring more employees (35,000 in the last two years alone) at the same time they are cutting hours.
That could be driven by the wage increase, but it could just as easily be driven by free market pressure to increase profit margins. Wages could've been left unchanged and they still might've gone this route because wall street is a bastard.