Not when they understand its limited and communal resource that they are being given unmetered access to.
Your argument just collapsed. Average people know exactly sweet fuck all about how these things work. Even most technical people don't know the details of how ISP equipment runs.
What we do know is that we haven't been charged per minute for our phone service or TV service for the past hundred years like the British have so why should the interwebs be any different? The internet comes to us through the exact same wires as our telephone and/or cable TV.
Its only bait and switch if they change the terms or product on you.
They did change the terms of the contract. Just as we're not all data center engineers, we're also not all contract lawyers and even if we bothered reading the fine print (which is unlikely,) most of us won't be able to spot the loopholes and predict all of the consequences. The fact that they retain the right to change the terms somewhere deep in their 47 pages of legal bullshit (that you can't read until you've already signed up, since they're certainly not printing all that so you need to look it up online!) means they're still changing the terms.
especially since only a tiny fraction would elect not to renew service under the new offer anyway.
Actually, a large fraction would elect to go to a competitor -- if there was one. This kind of bullshit tactic only works when you have a monopoly (or small oligopoly) in your market and your customers only have the choice of sucking up your bullshit or going without entirely, and the internet is a service that isn't practical to go without for the vast majority of people.
Should she have sued them?
No, but she could have. That's false advertising.
For damages? What damages?
And that's why they get away with this kind of shit. Its not worth it to stop them. You'd have to be on one hell of a personal crusade to invest lawsuit-level money over a couple of pieces of fish.
But just because something isn't worth the cost of fighting, doesn't magically make it right. The restaurant is still absolutely in the wrong in this instance and your sister has all cause to be pissed off at them. But unlike ISPs, restaurants have huge amounts of competition in most areas (even if you ignore cooking for yourself) so your sister can certainly choose not to go to that particular restaurant again.
Or if she's the kind of person that doesn't mind confrontation she could just choose not to pay since they didn't uphold their end of the bargain, or pay a fraction based on how many more pieces of fish she figures she would have eaten had she been allowed, or whatever. Now its up to the restaurant to decide whether its worth pursuing the issue.