To clear a few misconceptions right from start.
No, this is NOT broken.
No, this is NOT bugged.
No, this is NOT weird.
No, your camera doesn't stay behind your back when you move left or right.
No, it's not like "in Firestorm", its ANY Viewer. Stop crediting them for things LL did.
Most importantly, no, this is not something camera related as literally everyone thinks.
What's the issue? You start the Viewer and hit A,D or Arrow Left or Right and your avatar moves to your left/right. This is not weird this is the industry standard since... 20 years? Pretty much since 3d games became a thing. It's called strafing, although compared to games it does not look like the usual strafing e.g looking forward while walking sideways, yea that's SL for ya. Strafing is set to replace the rotational movement by default, why? Because every game that isn't a complete pile of shit does this by now, just a few special snowflakes haven't catched up with something that has been established back in doom and quake times, namely Second Life for instance. Second Life as such classifies as game, more specifically as MMO, and even more specifically as Simulation and Sandbox. It's a Sandbox MMO and it offers what is quite standard for said MMO type, third and sometimes also first person camera, those are in 99% of all cases set up to have A/D for strafing rather than the long gone turning. But how do you turn then? You use your mouse of course. We didn't get this input device for nothing but clicking UI elements, its there offering extremely precise movement in 2 directions, enough for looking around in a 3d world such as Second Life. You can simply click and hold the left mouse button on your avatar to enable "Steering Mode" in which... you guessed it, steer your camera, dragging your avatar behind (yes your avatar behind, it follows your camera rotation not the other way around). You can also simply hold your right mouse button anywhere that is not the UI and drag your mouse around to do the same (this also works after alt-camming).
But this guide is for those who simply cannot use what is considered the most efficient, most convenient, fastest and most precise, industry standard way of controlling your third-person-viewed avatar/camera.
The secret reason i changed this is to teach you something. I do this all the time. Everywhere. I do the thing that seemingly no one else has the time to do today. (Guess why all the names of graphic features are labeled with their correct names, it's so you can google them and get some actual information on them: Wikipedia for instance. NO ONE KNOWS WHAT ADVANCED LIGHTING MODEL IS GODDAMNIT)
So what you want is, you want to turn around using (most likely) the arrow keys, rather than strafing to the left/right as it is by default. This is actually very easy and i don't know why this even needs explanation but...
In Preferences - Keybindings, you can find several lists filled with controls and actions. It looks like this.
Action tells you what it does, function is the internal name of the action, button is the currently set key that will trigger said action and modifiers are extra keys such as CTRL, ALT, SHIFT. In order to change the strafing back to turning you'll have to find and select the entry that reads "Turn Left" / "Turn Right" bound to Q and E and modify them by hitting the "Change Selected" button (or double clicking the entry). A new small window will pop up. This new window displays the old keybinding at the top and the new one below, all you need to do now is press your new desired key you wish to use to trigger turning either left or right, in this case either A/D or Arrow Left/Right. After pressing the correct key and having it show up below, hit "Bind" and repeat the process until you changed everything you wanted to change. In order to prevent you from strafing and turning at the same time now, you'll have to select the "Move Left" and "Move Right" entries that have your keys bound you want to use and delete them so your keys are not bound twice to two different actions. You can check if you did everything right by pressing your keys and if you did everything right your keys should now do these:
That's it actually. In case you REALLY need help, here's a the entire instruction in a 10 sec nutshell.