Blergh~
I think the rule of thumb is that the most specific ruling takes priority. In this case the strict rule for specialist wizards would take precedence over the more general ruling from Use Magic Device.
I could honestly see it going either way. A specialist wizard could drink a Potion of Shield or use a magic item that casts Shield x times a day right? Or some kind of item with permanent Shield right?
Its never a good idea to try and divine what someone was thinking when they wrote these things up.
If I had to guess, and I'm trying to do the mental gymnastics to follow someone else's train of thought here, they don't want a wizard that can't use, like, evocation spells to ignore the restrictions and reap the benefits by using a wand of fireballs. But going in through the side with Use Magic Device, which lets the wizard function as a rogue or bard and try to get something to work by waving it around and shouting words like a busted Television Remote, doesn't seem to be a bad compromise. Since he can't activate it at will as if it was on his spell list, he has to make his UMD checks to get it to function like any other class.