You can use AdBlock for more than just blocking ads. In-page pop-ups, or the "Subscribe to our newsletter!"-style pop-ups that overlay a web page when you visit a site, are not blocked by default. EasyList, the primary filter list we use to determine what to block, considers these legitimate self-promotion rather than third-party ads.
If you would rather not see these, consider subscribing to AdBlock Premium and using the Distraction Control feature: About Distraction Control.
With Distraction Control enabled, you can then report unblocked distractions directly to us using our troubleshooting tool: How to use AdBlock's troubleshooting tool.