It's the way every terrain attachement works : lava does exactly the same thing. It does feel unintuitive, but i like the way it works, it gives more tactical option.
It's not a bug, but I agree that it's pretty weird in the case of smoke. For lava, spikes, and acid, I think it makes sense that they replace the 'difficultness' of the terrain with their own. (ie. spikes and acid are difficult terrain, but lava is not). But for smoke, it feels pretty unintuitive to me. It gets particularly weird when someone has Hover - because ignoring terrain effects with Hover means that difficult terrain covered by lava or smoke will once again be difficult terrain. I think that's pretty confusing. (I think it's only confusing because of the names of the effects. The descriptions of what they do are accurate; and I certainly don't think there's a problem from a balance point of view. I just think the names don't exactly match what the effect is in some cases. Obviously in 'real life', smoke wouldn't have any effect on whether or not a swamp slows someone down - and smoke certainly wouldn't remove lava or spikes!)
