I agree that the website is the thing that is broken. Their code looked like this:

  reportToTracker('userActionSubmit');
  submitForm();

The first line threw an exception. They should have surrounded that line with a try/catch block. And they should have tested against popular adblockers as well as popular browsers. But it's also within their right to not care about adblockers, and live with the consequences of upset customers. Unfortunately, very few customers will complain, so this becomes an externalized cost which the website owner never sees.

Practically speaking, if I install an adblocker on my friend's computer, and their mobile provider website stops working, then I personally feel some responsibility for making my friend's web experience a bit worse.

>Ad-blockers have a bigger market share than most browsers!

Browsers adhere to standards. Meanwhile, there are hundreds/thousands of filter subscription combinations that people use, along with several adblocking extensions.