Not sure about the AM fields and crap now. But originally it was only Anti-Magic shackles, and was in a way, a magic to supress magic. They were inchanted to rebound magic. So if you were to try to focus magic and break them, or attack, it would keep the magic within the mage/prisoner. Because a mage needs to concentrate to cast a spell...
Uhh, how to better explain it... The anti-magic shackles would cut the magic off, the mage would still have the magic, but they couldn't do anything with it, because of the enchantment on the shackles as sort of a... concentrated force-field of magic. That's why you would have to find other ways to break them or get them off, magic wouldn't help in this case.
That was the original idea, anyways....