It's not directly illegal in the state of Ohio. What that saying from the ride hosts comes from is the rider safety act which in paraphrased terms says riders my obey all posted and verbal policies of the park. So technically since it is a rule that is posted and verbally communicated to guests, you could twist an argument to say cell phone use is illegal. However, please don't think the authorities are out looking for all of the violators of a park policy. The law is in place for liability reasons.
In the end, the ride hosts are being snarky when they say that.
Good to know - because I know I heard multiple ride ops say "It is illegal in the state of Ohio to use a phone/camera while the ride is in operation." My last visit to CP (which was a few years ago) and at KI, (more recently) I heard it on more rides than less, which is why that stuck with me. I was also interested in it because, it seemed awfully specific. Thanks for the clarification. Could be a useful piece of litigation regardless to delve a bit deeper into regarding the aforementioned issues.