This is based, as I believe, on the commandment not to take God's name in vain. Similar piety is evidenced by Jesus Himself when he uses "reverential periphrasis" to avoid saying the name of God. Even today people will speak of the "will of Heaven", when what they mean is the will of God.
The avoidance of spelling God or Christ completely is a sign of pious respect. Certainly it can be disagreed with, but to suggest that the person who does so is "ashamed" of God, as I believe another poster did, is to miss the point.