They look at the extra 50 people that sign up for whatever or install the app, but don't think about the 500 people that clicked the back button and resolved to never go to their site again.
A better way of handling this, IMO, would be to let the readers see (some largish number to be determined) pages from the site and then ask them if they want to install the app. That way you're directing the pitch toward people who might actually be interested in seeing continuing content from the site.
