Outside of writing failure, which never happens 🙂 , or xander stealing government minie mikes which are not shown, Ed's right on. It is the only plausible explanation. Maybe we can flesh it out a little more, too.
Maybe Buffy has some left over uber power from 'earshot' or when she had that ability it 'strengthened' her mind enough as far as psychic powers go that she can initiate a conversation with someone receptive and capable like Willow, but someone like Xander or a non-psychic could not?
If it was always on/without filter, and always open, I'd imagine it would drive Willow crazy, like it would Buffy in Earshot, or at the very least bother her.
Who knows maybe it did?