I've stopped complaining about this when I realsied the ads from a couple sites I enjoy don't show up in landscape mode. "Rotate your phone please", nice try!
I've had those popups numerous times and it makes me irrationally angry. Why does an ad get to dictate how I hold my device?
Because you're not paying for the content, so the publisher is desperately trying to get you to click on the adverts which are the only way they can make money.