Facebook acquired the app in question, so I'd imagine that many of the developers working on it joined before it was part of Facebook. As for why they'd be oka

Facebook acquired the app in question, so I'd imagine that many of the developers working on it joined before it was part of Facebook. As for why they'd be okay with working on it -- well, the world is not so black and white. I think many people would consider a free VPN that collects statistics on what apps you use to be a fair tradeoff. There are many circumstances where being able to hide your activity from your government / employer / ISP / whoever is more important than some random third party knowing what apps you're using.