This is based on the fact the Ice is in the water already. Glaciers are not in the main body of water yet until it has melted. So the main body of water has yet to account for the melting Glaciers. :)
Glacier melts - Adds to the ocean level.
Also, Oceans evaporate, adding to the atmosphere which ends up raining/snowing back onto land. If the Glaciers didn't melt to add BACK to the oceans, we would see lowering Ocean levels (not by much, just enough to make more arguments).
This is always forgotten in the arguments.