We live in a finite world. Therefore "Unlimited" hosting is not possible. Web hosting is a very competitive business. Web hosting companies are doing all they can, including giving out free migration services and free Google Adwords credits, to win new customers. Because "more if often better" in consumers' beliefs, "unlimited hosting plans" became a popular marketing tactics in mid 2000's (and, if I recall this correctly, BlueHost was the very first that started this).