I think that’s just because that’s a more natural resting starting position. Which is why you just really see it at before the start of the freestyle when they are just waiting for the music to start. Also even though they start in that position the concept of letting some slack out, or at least releasing the yoyo from your grip then throwing, first still holds.
It’s not as natural(also wastes a bit of extra time as well) to bring it to that position after every throw mid freestyle.
I think because the ‘teaching’ way is much less likely to result in the yoyo hitting the ground. It’s also slower and more controllable, especially if you’re teaching a kid starting out with a responsive yoyo.
The ‘pro’ way requires a much quicker action requiring more coordination and also tends to result in a much faster breakaway. I’ve been teaching my 12 yo daughter and I know she wouldn’t be able to use the ‘pro’ breakaway method yet.
I learned this by watching YouTube videos frame by frame - it was then I realised that these advanced players were releasing the yoyo as soon as (If not before) they began the motion.
The YoYoJoe1 channel is particularly good for this - his camera/lighting setup is pretty good, making the frame by frame much easier to see. Brandon Vu does everything so fast and his video quality isn’t the best, making it much harder to see what he’s doing in slow motion.
I still think the ‘teaching’ method is pretty good for kids, especially with a responsive yoyo. Much less chance of it hitting the ground or smacking them in the face 
Breakaway with intent or don’t breakaway at all! 
i’ve always just turned the yoyo sideways in my hand and acted like i am turning a doorknob really hard. it’s always worked for me.
I just focused on the flick of the wrist.
Of course people can throw however they want, there are no rules but I always find it weird seeing people who can do tricks far more advanced than I can who still throw in that weird way beginners do.
My throw varies throughout the day. I’ve broken my TH wrist twice so my pro-ish throws eventually turn into full on, super exaggerated noob throws after a long session and that’s just fine with me. The end result is the same as far as I can tell anyhow. I don’t see a right or wrong way as long as it gets the job done.