The short of it is that Google penalizes pages if too big a percentage of the above-the-fold content is filled with ads. So if you want your pages to rank highly, you need to keep the ad real-estate reasonable. However, once someone gets to your site via a highly ranked landing page, there's nothing stopping you from getting a user to click on an internal link that takes them to a much more ad-filled page (which doesn't itself rank highly on Google).
The short of it is that Google penalizes pages if too big a percentage of the above-the-fold content is filled with ads. So if you want your pages to rank highly, you need to keep the ad real-estate reasonable. However, once someone gets to your site via a highly ranked landing page, there's nothing stopping you from getting a user to click on an internal link that takes them to a much more ad-filled page (which doesn't itself rank highly on Google).