YKW,
In your 1st two examples, the kick has been blocked/redirected, so there wasn't an opportunity for you to retract the leg. For myself, most of the time ( but also depending on the kick, the level, etc.), I prefer to pull it back. Most of my round kicks, except maybe to the leg, I pull back, for example. Although I've sparred with some MT guys, I've never trained it, so I'm not going to try to mimic that style of kicking. And as for side/back kick, besides the risk of your leg getting caught, if you just drop it straight down:
1. It creates the habit of pushing with your kick. IMO, in both bag work and sparring, retracting my side/back kicks after contact transfers a greater 'jolting force' into the target.
2. Dropping it straight down can leave one vulnerable to a foot sweep as it's about to touch down.
3. IMO, it leaves the leg hanging and the groin open a split second longer. Sometimes an opponent who is countering can ball-shot you, even unintentionally, if you leave it exposed too long.
Anyway, that's my .02.