"Unlimited" is just a holdover from 56k modem times. They offered a number of minutes per month for the basic subscription fee, and over time as AOL and MSN and the occasional third leg battled it out for customers that number increased fairly consistently. Pretty soon it was $20 for unlimited internet. 5k transfer per second, 2592000 seconds, that's 12960000k or 12656m or 12GB. It would be impossible for you to download more data than that (we'll ignore compression since the limit ignores compression, and today's compression is better if not eactly the same).
If you consider your bandwidth as above, there is an upper limit on your current transfer per month, which is the unit by which you're paying. So it's imposible to offer "unlimited" access is there is a bandwidth limit. $20 for 12GB then would be maybe $30 now - for a 12GB limit? 24GB would probably be worth $40, unlimited would be pretty expensive.
So even limited, we're a lot better off. If we understand there's always a limit, that is, and if the ads quit saying "unlimited". The dial-up days are over, we're buying in bulk now.
"Your mother was a hamster, and your father smelt of elderberrys!" -- Monty Python and the Holy Grail