Earlier quoted context omitted.
This is typical marketing-speak. I am becoming temporarily deaf whenever I hear it. Read it the following way: We have a reason to do it but we don't want to tell you so instead we will tell you a little bit of righteously-sounding bullshit so that most of sheep base is satisfied. Yes, that's "sheep", because you are not our customers, our customers are those companies we sell your information and your focus to.
No, this is their reason, just from within their distortion field. This is helping the user view photos, because the user should be logged in and using the app, the app provides the best experience and this web interface is a distraction that gets in the way of that. Edit: I guess it wasn't sufficiently clear, the second sentence here is not my own personal opinion.
Nonsense explanation means just that users' interests were not factored in the decision.