Hypersmurf
Moderatarrrrh...
Granted, that is a major artifact, so I guess it's to be expected that it could break some "rules".
Indeed.
That said, it probably should be limited. It seems odd to me that the restriction wasn't written into it, as they made sure to restrict all other effects that give an enhancement bonus to items in the core rules. Bleah.
If they intended it to be limited, they may have expected that people would apply the ruling from the Magic Items section, and be surprised to find out people don't do it that way.
If they intended it not to be limited, they perhaps should have made a note to that effect in the feat - a Special/Normal thing like other feats have.
Special: The enhancement bonus may be greater than +5 if the character's Cha bonus permits.
Normal: A shield may not have greater than a +5 enhancement bonus for either attack or defence.
-Hyp.