I've been surprised by the lack of PFD's in river SUP, and I think its a bad idea. I also think that the marketing has something to do with this. Seems like every ad I see is a chick in a bikini paddling without a PFD, or any gear for that matter. I understand that PFD's obscure important things that are good for sales, but thats another story.
I personally don't agree at all with the concept that paddlers should pick and choose when to wear PFD's or not. Just because dumbass tubers don't wear PFD's doesn't mean educated paddlers shouldn't. Also, just becuase ocean surfers don't wear PFD's doesn't mean we shouldn't. The ocean is more bouyant due to salt water density, and sometimes folks wear bouyant wet suits. Surfers also duck dive waves, which would be harder with a lot of bouyancy. SUP in the rivers is different though, we have fresh water that is less bouyant, rapids that make extended downtime dangerous, and eddies and holes that have aerated water that can hold swimmers for a long time.
I also personally hate the concept of "I'm good to go on this type of water, so I don't need all the safety gear I would otherwise wear". This is a very limited and self centered view. You might just need that PFD to rescue a drowning kid in the lake. You never know. I recall paddling gore in the winter and one guy had shorts and noted "I'm not going to swim". Well thats great, but what happens when someone else swims? Pretty weak logic in my mind. I now know to avoid these kinds of guys like the plague becuase they are unprepared when the shit hits the fan, and it can hit the fan anywhere, at anytime, to anyone.
Another issue I see is that of setting examples. I'd advocate that expereinced paddlers should try and set examples for wearing the appropriate protective gear, so that novices can have examples of what well prepared river runners do regarless of craft.
Finally, whats the benefit of no PFD? Its like a seatbelt... almost zero downside to wearing it, and massive upside if and when you need it. The other point is that you NEVER know when you need it. You aren't that smart and you don't have a crystal ball. I hear this same arguement from snowboarders not wanting to wear helmets, and from rock climbers not wanting to wear helmets, but in my opinion its pretty dumb. Wearing protective gear that is comfy, cheap, and easy to use that prevents low probability but high consequence fatal injuries is a no brainer in my mind.