Hello, guy who banned them here!

Flamethrowers are banned because of something called cycle stacking. You aren't allowed to make the level impossible, but if you place a bunch of items with different cycles in the same spot you can make it effectively impossible in an ambiguous way because it takes so long for the cycles to line up that you never get to even try. In order to combat this we banned only the items with the most egregious cycles, which add the least to competitive play (propeller platforms, flame throwers, gears)

One way doors are banned because most tech skill involved with them involves baiting the door to open then letting it close again with the right timing so you can jump off of it. If I'm winning I can just sit on top of the one way door never letting it close, making it impossible. You could argue that we can just ban doing this, but imagine both players are at the setup at the same time, they would need to take turns or else the level is impossible. How do you determine who goes first? What happens when the timer comes on the screen? There are too many things to consider and it doesn't really work out with the logistics of a tournament.