It's deceptive advertising, which is illegal...if someone raises the flag and says, "This is deceptive and this is why." Anyone else remember the toothpaste ads (brand name omitted) where they'd dip a piece of chalk in some green substance, break the chalk in two to show how far the green substance had penetrated? That was supposed to be indicative of how their tooth-protection whatever-it-was penetrated into teeth. That was challenged under US advertising law because while it might have been similar to what occurred with their product, it didn't occur that much, and didn't occur in all instances.

It's the same with unlimited phone talk time, or unlimited bandwidth (and that's why so many "unlimited" providers have all the fine print: it's unlimited, but there are other ways they can stop people from putting a huge burden on system resources. But the word "unlimited" really reels people in. So they use the term, and either because no one's challenged properly or because of legal technicalities, they aren't banned from or penalized for using that terminology.