If they limit the rate of speed it's technically limited which really makes me wonder how they legally can say these things. I guess it means in a lot of cases it's like Comcast where they also limit the data a month perhaps but dang.
In the language of marketing (in the USA at least) the word "unlimited" means "limited".
They mean that they're not going to limit the total amount of data that you send/receive beyond the natural limit implied by the maximum rate.
When a movie subscription says unlimited movies, we know they're not suggesting that they can break the laws of time, just that they won't turn you away from a screening. It's pretty normal language, used to communicate no additional limit, which is relevant when compared to cell phone data plans (which are actually, in my opinion, fraudulent) that shunt you to a lower tier after a certain amount of usage.