There's so much more to it than that. They could slow down the bandwidth of websites they didn't agree with. Like, if a staunch raving democrat owned Comcast and just decided that the bandwidth for Fox and all the blogs like Drudge would be slown to crawl they could do that. Well, they could do it now, but with neutrality it's against the law. That example wouldn't bother me, but I'm an American and believe in equality and free speech.
the whole bandwidth slowing/throttling thing is just the means of extorting payment from the sender as well, since currently, i nor the providers, know of another way of forcing the sender as well to pay for delivery, .
or it could be used to force the consumer to deal only with senders whom said providers have an equity interest in. (ie a Comcastflix or ATTflix service, instead of Netflix or Hulu or whatever).
regardless, it's a means to extract revenue from not just the consumer/recipiant, but the sender as well.
as for your hypothetical and a literally zillion others, the prospect of blocking or throttling for non monetary reasons, regardless of motive, is also very much a reality in a non neutral internet world
after all, "no rules" means "no rules".
as for the case in question, throttling for functional rather than monetary, idealistic, or other reasons, is a different story, and that is what AT&T was doing here.
i see what AT&T did, and was fined for, as falling under false representation of terms of service, rather than any net neutrality type violation.
they were throttling a particular consumer/customer, not a particular sender.
that said, absent any NN type rules, one could see, or even predict, a scenario where a customer who had passed their usage barrier, (even if spelled out in their service agreement), being throttled for all incoming traffic OTHER than that which was sent by an entity in which AT&T had some equity interest in, or was receiving payola from.
were that the case, where the customer themselves rather than the sender, was throttled for excessive usage, but some senders with whom AT&T had a relationship with were exempted from said throttling of said customer, that then would be a NN issue.