It's not really a fee. In some cases they don't offer the lowest category, eg BF, for selection. The cabins you can select start with the BE category so show as a slightly higher price. This is stupid, IMHO, as some people actually want to select BF cabins.
And when there are only a few cabins in the next available category to select they may get snapped up quickly so the fare for selecting is then the next highest category.