I know this has been raised before, but I'd like to bring it up again. Even though this may be a low-priority issue, I'd rather bring it up than forget it completely.
While the one-way arrows displaying over objects doesn't affect the level functionally (i.e. they don't display on objects when playing the level), this can be very annoying and distracting while one edits their level.
Low priority, and probably a lot of work to fix. Next time I'm working on the editor I'll see if there's any simple way to fix it that I've overlooked, but if only complicated solutions exist, it probably won't be fixed for a while. I do, though, fully agree that it should be fixed eventually.
I noticed that this doesn't happen if they actually have the "Only Draw On Terrain" flag set. I looked into why this happens, and it turns out - it's simply a matter of that the editor draws such objects before any others. I modified it so that it also includes one-way-arrows in this "draw first", and the result - this bug is finally fixed. :)