Forced registration to read it would be my guess. Nothing else against it really personally. Some people just don't like having to create an account to (passively) participate in a community. I read hacker news for well over a year before I decided to create an actual account for posting.
If I feel I reach a point I can contribute something useful to a community, that is when I create an account. However, I think forcing people to create an account either detours some or nudges others to participate more (whether they have anything useful to contribute or not).
Even then, creating an account to post to a community is an absurd idea in the first place, why do you need my email, name, dob, and social security number to post on a bloody internet forum and contribute content to your website?
There is your usual suspects: combat spam and the anonymous coward effect but even then there is many ways to combat these without invading my privacy / lessening my security online.
I think it mostly comes down to lazy analytics and shitty metrics. "we have xx registered users",90% of those are made up of shell accounts and inactive users but at least the numbers sound good.
This app thing is just the new incarnation of shitty metrics and the same old story of
developer: oo new tech, me want to makey makey.
boss: can you justify it, isn't our mobile site fine?
boss: ok ok, jesus, fuck. Just, we'll have to make sure users actually install it though, or we'll be out of the job.