As 005 said, I always thought of the shields on Hoth as being "above the ground," intended to stop orbital bombardment, with nothing making contact at ground level. This is why the walkers land farther away and simply approach by ground. Unlike the spherical Gugan shields, which protect from all sides, the OT's shields seem to have the capability of being isolated to face a single direction (as seen when the rebel pilots switch their defelctors on "double front" during the Death Star attack in ANH).