On a crop body it gives you a 180 view at all focal lengths? And on a FF body it allows you to do a fisheye at the lower end and 180 degrees only at 15mm?
You're getting 180Â° at all focal lengths on all sensor formats. The difference is that on non-FF cameras, you get 180Â° diagonal AoV that fills the frame (at 10mm for APS-C and 12mm for APS-H). If you go wider than that on those formats, you get vignetting around your image (i.e. part a circle). On a FF camera, at 8mm you get a circular fisheye, with 180Â° coverage all the way around (vs. just diagonal, which is the longest dimension of the sensor).
So basically, you get 'full frame fisheye' coverage for any sensor size (as opposed to the EF 15mm f/2.8 Fisheye, for example, where you get 'full frame fisheye' only on a FF sensor, and a cropped, less fishy version on smaller sensors), and on FF you also get the option of a true circular fisheye.
Below I took Canon's example of the different coverages and superimposed the APS-C and APS-H sensor sizes onto the FF circular (8mm) setting, to give an idea of what vignetting would be like. On the lens itself, Canon added a zoom range limiter for APS-C and APS-H to allow you to easily avoid going too wide.