It's a common theme with fly fisherman... I have ever understood it. Doesn't matter if they are single hand fishing, spey fishing or the like, they just like to get in the water and go for it. Whenever I go steelheading on the fly with with fly fishing buddies, I'm constantly reaming them to stay out of the water. I catch a tonne of my fish on the fly in the slower water in close, on a dry line, just as my fly swings into "dead water."
Just the other day I razzed someone I knew who posted a video of them catching a fish, in water up near their chest (literally the middle of the river). The reply was that the method (wading out to your chest) is needed for Czech nymphing. Now I understand the basics behind it, fish in close and work your way out after covering the water in front of you. Still can't figure out though, why they need to get that 1 fish in the middle, when they could instead cover more water only wading out knee to waist deep, and working down instead of out. From personal experience, and observations of trout, salmon, steelhead and bottom fish, they all prefer slower water closer to shore, rather than the middle of the river.
On their own personal river with no one else fishing, I guess wading out to the middle is completely fine. I feel like it's definitely poor etiquette to wade out to where fish are sitting on popular rivers. If they feel the need to go knee deep, have at er, but waist deep or more is uncalled for.
Plus add on the fact that you are now standing in the middle of a river, in current... I don't care how good of a wader you are or "think" you are, things can happen, and do happen fast, even in slow moving water. I don't think I've seen a single one of these fisherman wading that far wearing a life jacket... If I was doing that sort of wading/standing for long periods in deep current, that would definitely be something I'd be wearing.