I suppose it's a case of we spell it how we say it. It's okay... I think the Euro version is cute. Much more appealing to hear some kid yelling "Mum" through Wal-Mart as opposed to screaming "Ma!" at the top of his lungs. Guh.
Another thing- maybe it's just me, but I don't think our mouths readily form that word. It seems odd and forced. Probably because we don't use it.