Actually, they're not exactly doing it for nothing. They're providing a service in exchange for your lists of relatives, for whom they will later be baptized in proxy to make posthumous Mormons out of them (if they choose to accept it in Heaven, etc etc etc). Baptizing themselves for your dead relatives is a type of missionary work for the LDS.
If you don't want your great-aunt Martha to be placed on any LDS posthumous mailing lists, don't accept any free services from them.
Sorry, but this is not true. You are under NO obligation to give them anything. If you do not want to share what you find you can just go home and leave nothing behind.
You do pay 23 cents for a scan and copy of old records, a nickel for a copy, and a smile to the lady behind the desk who helps you out. Those are all less than cost.
They do collect their 10 percent from their members, of which I am not one.