At present, only three SE sites carry commercial ads: Stack Overflow, Server Fault, and Super User. They are banners placed below question title, in the sidebar, and between answers.
Consequently, these sites have the 200-rep privilege at which most of these banners are no longer shown.
There are no commercial ads on other sites yet. Although SE plans to expand ad sales to other sites, listed below:
Android Enthusiasts Arqade (video gamer) Ask Different (Apple) Ask Ubuntu Database Administrators Drupal Game Development Programmers Security SharePoint Unix/Linux Web Applications WordPress
That the list of privileges on Meta Stack Exchange and on MathOverflow include the 200-rep reduced advertising privilege is simply a bug. In case of Meta this seems to be a bug-by-design. Who knows how it came about on MathOverflow. If someone complains on their meta, the bug might be fixed. Or not.
I emphasize that the above is about commercial ads. There are other banners present around SE networks, some of which are intra-network site promotions (example below), and others are Community Ads. These are not disabled regardless of reputation.