I simply don't understand it, so I'm hoping someone can explain it.
You can hold a non-magic shield just fine. You can hold a magic weapon just fine (dual-wielding), so why not a magic shield?
It just seems to me to be an unnecessary and unfair disadvantage. What am I missing?
It's totally fair.
As a general rule, each PC can have the same number of different types of magic items.
There are some exceptions, all of them dealing with items that give enhancement bonuses to attack and damage:
1) Clerics who get their Holy Symbol for free (i.e. as an additional free slot)
2) Staff Wizards who can combine their weapon with their implement
3) Rangers who use two weapons
But even in these cases, it's not as if they are getting more power. Sure, the Cleric can use his magic Holy Symbol or his magic weapon whereas the Fighter just uses his magic weapon, but since they are both used for attack, it really means that the Cleric is both at a disadvantage due to having to own two magic items instead of one for the same gain, and at an advantage (typically with the daily power of the extra magic item). Ditto for the other examples.
They are all bonus items to attack and damage powers. And, this is the important part. Typically, the magic from the two items cannot be used at the same time. The user has to decide to use one or the other on an attack (the exception to this is a two weapon wielding Ranger and even in that case, each roll has to use one or the other).
So, the off hand can be either a magic shield or a magic bracer. Each PC has to decide what he will put in that slot. The only way to get bracer magic while using a shield is to use a mundane shield. That's totally fair. It applies equally to all PCs.