It was probably a legacy handed down from Fishermans Paradise, which still has a no wading rule.
The reason it was instituted at the Paradise was that the fishing was so crowded, literally elbow to elbow, that allowing wading would have caused conflicts.
At one time the no wading rule on Spring Creek extended way up to near the pistol range. Most of that section is wooded, not groomed, so it was very difficult to fish it without wading. So they changed the no wading to just the groomed stretch near the hatchery and parking lots.
The special regs area on Kettle Creek used to have a no wading rule also. That is wooded, not with groomed banks, so difficult to fish from the banks. Back in the early 1970s the local WCO would hide and watch with binoculars, then fine people who stepped into the water.