Actually, it probably has nothing to do with monopoly status. In some jurisdiction (such as the UK, as I recall), you cannot provide a product without some minimal warranty -- such as merchandisibility, i.e., that the product works for its stated intended purpose.
Fair enough. I put that there to account for the difference in the GPL, but since products covered in the GPL usually require no legal consideration in exchange, the GPL can get away with the "no warranty" clause.
BTW, MS used to have the "no warranty" clause in its Eula as well--until the law forced them to change it.