If a user pirates content on the internet, the ISP is not seen as complicit because it is just a dumb pipe, am I right?

Would that still hold if they inserted ads?

Without consent, you're absolutely right. But if (as in the case) the ISP not only obtains consent but also gives significant discounts in exchange for it (and owns the pipe through which the content is delivered, which may give them rights to modify data that a browser plugin company doesn't have), my guess is that AT&T would be in a better position.

The only redeeming feature is that AT&T is giving a discount for the service. I suspect they'd run in to more lawsuits about their monopoly status if they forced that decision on all consumers.

However, the plugin companies aren't in an ambiguous situation at all: someone who requests the page is explicitly requesting it to use the content to run a program which renders an image from it, and is under no contractual obligation (from merely making an HTTP request) to view the entirety of the content. They can neither complain that it's being processed (the browser inherently does that, to make an image from the text), and they can't complain that the end-user is failing to uphold their end of an agreement.

Because it's a user chosen process, and because of how content on the internet is relayed, there's no reason that the plugins are on any kind of shaky ground. (One notable exception would be interacting with dynamic content to bypass a paywall. There likely are other exceptions, but these come down to bypassing some kind of mechanism, not merely failing to load or render a particular portion of the sent content.)