Since it's not logical, i'm going to play it with the fact that figures do not block line of sight of sweep. More fun, less complicated.
"Stupid is what stupid does"
These are the kind of comments that are likely to trigger people being defensive. Implying that they, the way they think, or the rules they play by are "illogical" or "stupid". Be careful of your language or be prepared for people to take offense.
You're only talking about rules.
I'm talking about logics.
It's been discussed MANY times before, but this is an incredibly slippery slope, especially in a discussion about the rules as written, which this thread is. You can come up with all sorts of "logic" to describe things, but that doesn't make them true. ("Logic" is not necessarily truth, it is merely a pathway of going from one idea to another via some other idea or set of ideas, and a person's logic may be objectively wrong. It may also be right, but for the wrong reasons.)
In general, it is a much wiser plan when trying to tie reality to rules (which I don't think you should do anyway, more below) to find a situation in reality (of which there may be many) that reflects the implementation of the rules (of which there are few) than the other way around. Example: Instead of immediately jumping to the first assumption that comes to mind with "Well this doesn't make sense because if (insert assumption here), then it'd work this way, so I'm going to change the rules" invest a bit more thought and search for some assumption that fits this: "Well, if we think of it like (insert another assumption here), then this rule actually makes much more sense."
That said, everyone should remember that game rules for almost any immersive game are an abstraction of reality, not a model of the universe's physics. In Descent, Range and Line of Sight for attacks aren't always hard and fast rules about "Can you shoot an arrow this far" or "Can you see the target". They represent an added difficulty based on distance and obstructed vision. I might be able to shoot an arrow twice as far as my target, but it doesn't mean I can hit my target with any consistency. I might be able to see parts of a target behind a tree, but the likelihood of me hitting the target is so small that it's pointless to try. Once you realize that rules are NOT physics and are instead an abstraction intended to create balance in the greater picture that is the entire game, and that changing such rules may have unintended consequences (as Varikas pointed out), you'll, in general, be much happier with many games.