How can anyone agree to what OP is saying. It's silly.
- Standing in a cloud of toxic gas = breathing it in and suffering from it
- Stepping out of the cloud = still taking damage from it's toxic effects, but at least you aren't breathing it in more, causing further damage
- Standing on a hot surface = it ignites your clothes and you're burning, but you're also standing on the hot surface, which is painful as well
- Stepping off of the hot surface = You're still burning, but at least you aren't standing on a hot surface anymore
Seriously, imagine a room filled with gas, when you leave it, you'll still be hurt by the gas, but it wouldn't be as bad as staying inside. Imagine being a piece of food in a frying pan, when you are taken out of the frying pan, you're still hot, but at least you're not being heated up even more by the frying pan.
Why is this not intuitive for you?
Kriss, before criticizing me like that, at least make sure you're understanding what you're responding to. Maybe I should be asking why the original post is not intuitive for you, because you seemed to have missed my point entirely.

I will, however, make an effort to break it down again.
The problem is not that you continue to take additional poison damage from standing in a poison cloud for additional turns. For example, if you stand in a poison cloud for 3 turns before the cloud dissipates, I fully expect you to take poison damage 3 times - once for each turn, and then probably additional damage for a few turns after that as the poison debuff on your character wears off. Nobody is suggesting anything is wrong with that.
The part I have a problem with is when you take poison damage *ten times in a single turn* because you walked through a poison cloud rather than remaining perfectly still in it, as if each step you're taking (probably in an effort to move out of the cloud) should poison you all over again. Poisoning you for the amount of time you spend in a cloud = perfectly reasonable. Poisoning you for the amount of steps you take through a cloud, however, makes no sense.
Fire has the same problem. For some reason in Divinity, you'll take less damage standing in a fire for 3 turns than you will if you attempt to take 4 steps out of the fire in 1 turn. That makes no sense.
Now, there are examples where the above behavior of movement versus time does make sense. For example, every step taken across a frozen surface should (and does) introduce a chance of being knocked down. Every step taken across an electrified surface should (and does) introduce a chance to be stunned. But fire and poison damage should occur over time, not over distance traveled. Sure, the chance to be poisoned or set "on fire" should occur with each step, but once you have the debuff you shouldn't be taking additional damage with each step - only with the passing of more time.