Well there are negative creature enchantments, but those make more sense, because flavor wise it is understandable for a mage to imbue an enemy creature with some sort of ill effect.
Negative equipment doesn't make much sense: flavor-wise you are just handing items to your creatures. Clearly an enemy creature wouldn't trust you, flavor-wise, so thus there is the game rule. On the other hand, Control Circlets and Cursed Equipment sound appropriate.
There are some cards in Mirrodin, like the Vulshok Battle Master, which create situations where a creature can be equipped with an opponent's equipment, but these make sense flavor-wise too because theft of property is conceivable.
Regardless, there are some equipment with drawbacks, and conceivably they could be abused if equipped on an opponent's creature.
In the end, however, don't count ideas like Control Circlets and Cursed Equipment out. They're likely to make more equipment beyond Mirrodin block, and the flavor of such items seems fitting. All they have to do is add text on these equipment that say they can equip other players' creatures. Don't think it'll happen? Well you're not allowed to cast creatures outside your mainphase either, but that still doesn't mean Benalish Knight can't be played at the end of my opponent's turn.
