Good Eats actually had something on this many years ago, as effectively turkeys aren't just water, so it doesn't freeze at the same point at water.
Copying/reformatting from the Good Eats Fan Page transcript of Romancing the Bird:
Chuck: Just look at this. The USDA recognizes three distinctive market varieties of turkeys. The first is frozen.
Alton: Meaning below 32 degrees?
Chuck: Only turkeys cooled to zero degrees or lower can be labeled frozen.
Alton: Well, uh, set me straight here, Chuck, but I thought 32 was the freezing point.
Chuck: For water, sure. But the moisture in meat isn't just water. It's full of all manner of dissolved solids that effectively reduce its freezing point.
Alton: You don't say. Okay. Thanks.
Chuck: Oh, next at 1 to 26 degrees—although most manufacturers shoot for 24 to 26—are refrigerated birds.
Alton: But frozen are so great, why bother?
Chuck: Because refrigerated birds aren't as rock hard as frozen so they don't require as much thawing time. And grocers don't need as much freezer space.
Alton: Is that it?
Chuck: Oh, no, no, no. Fresh turkeys are a whole other thing. Fresh turkeys can never go below 26 degrees.
Alton: But that's still below freezing.
Chuck: Not in turkey.