It's their website and they are responsible for the content that goes on it... They don't get to just pass the buck onto the ad network when they are specifically making a choice to use that ad network, and profiting from it at the expense of their own users getting subjected to screen hijacking ads constantly when browsing on mobile devices.
It's a problem, and it's on their website specifically. From there, they can either work to address the issue and stop pissing people off who use mobile devices to browse their site, or start risking decreased traffic from the largest segment of the people browsing the internet.