Hey Jonny,
I think it is because Joe is using a fairly strong 'anchor point'. The fantastic camera views here clearly show his palm anchored at the top of the bridge, and it doesn't seem to move from that point throughout.
However, if you look closely, you can see that whilst he angles his wrist to move across the strings, he still seems to rotate the angle of the pick so that the pick is still striking the strings at a 90deg angle.
I suspect that this is a natural thing that Joe does without even thinking, and that indeed if you asked him this question directly, he couldn't explain it to you. It is probably something that hours and hours of playing and practice has instilled quite naturally.
I have heard that Joe has quite a soft touch when playing, but when playing acoustic, you have to 'dig in' a bit more to get the guitar to respond in certain ways. He probably has a very different technique when playing his electrics.
I am guilty of 'floating' my right hand too much when playing (especially on my acoustics), and am working on finding a nice, comfy anchor point that I can use as a base to improve my own picking technique.
JBLP Gold Top #129 - redubbed "#1 in Oz"