> they are simply detecting user agents with ad-blockers The browser's user agent string doesn't change based on plugins installed, or at all--unless end user

> they are simply detecting user agents with ad-blockers The browser's user agent string doesn't change based on plugins installed, or at all--unless end user changes it purposefully. The ad-block detection is likely and usually a separate script that runs an interval check on the existence/visibility of ad-related DOM elements (which would be false when hidden/removed by a plugin or even the developer tools)