Its an implication of the target market, and it goes hand in hand with facebook itself: if you’re looking for casual hookups, you probably
dont want to be openly exposed to friends/family. But if you’re doing something far more wholesome, like looking for a full
relationship, then its perfectly fine for friends/family to be aware. And facebook hosting a dating app would be totally unnotable if there was no chance to for friends/family to find out: the whole point is that fb can filter onto your friends, and friends of friends, and so on, to naturally limit your possible matches to something far better than “kind of close to you”.
The friend-links allow you to verify the target human yourself (ask your chain how they are), it limits misbehavior (you’re sending dick picks to your friend’s best friend, not a random stranger), you’re naturally liklier to share a social circle or taste or whatever (you’re friends with the intermediate person for some reason).
So it has to be marketed and tuned towards something that is palatable to your broader social circle, because they are, more likely than not, going to find out.
And while you might accept casual dating, and people often don’t care, its still a broadly negative thing to be going around with. The general expectation (at least in the us) is that eventually you’ll stop, and have a serious relationship, and you don’t really go around showing off your casual relationships (at best, you pretend externally that its possibly serious until the day you break it up), and you certainly dont go around telling people you’re currently looking for a fuckbuddy (and your friend might be the one!), at least not directly, and certainly not broadly.
You don’t want to find your teacher on tinder/grindr, and you really don’t want to find your friends on it. (As always, in general, and especially post-teenage, and a US opinion)