There's a theory that certain emails scams are so obvious and easy to spot because that acts as a self-selection mechanism. A person who sees the obvious scam and immediately recognizes it as such was probably never going to fall for it. The ones that respond in spite of all the signs tend to be easier or more lucrative targets.

I could see forcing people to download an app just to see the content as operating on a similar (but not 100% analogous) principle. The type of person who willingly installs the app to see the content (without knowing if it was worthwhile/relevant beforehand) may be exactly the type of person that they prefer to join their site. Perhaps they are easier targets for marketing, less likely to understand /complain about the ramifications of changes to the site that are user adverse, care less about privacy, etc and that makes them more lucrative?