Sorry! I've been reading and writing the wrong verb several times.
When Joefarebrother wrote "right but you're revealing them *from* a private zone" I misread and thought he was talking about discarding, not revealing. (The emphasis on "from" confused me.) And then in my last post, I even wrote "reveal" when I meant "discard". :p
So I have to start again. Yes, now the argument makes more sense. Hmmm. I guess I was thinking that there's a special rule for when-discard cards that you have to reveal them, even Village Green, Weaver and Trail, which are moved to play for all to see. There's no such rule for Sheepdog etc.
Actually, Tunnel is the only "when discard" card with an explicit "reveal", for accountability reasons. The other "when discard" cards (Trail, Weaver, Village Green and Faithful Hound) do not have it, since they already become public by being played/set aside when you use their "when discard" ability, just like Fortress becomes public when trashed (for a single moment).
So discarding in general is different from gaining/trashing in terms of "publicness", though it's debatable whether this is a good reason for deciding what should be a Reaction - as discussed, all "when discard" Reactions specifically make themselves public while reacting. There's also the case of Patron's "when reveal" - it's triggered from a public position by definition, yet it is a Reaction.
I do think that without the colouring, it would be easier to miss e.g. Weaver's ability with a Minion discard, than to miss Fortress' ability when trashing it. But that's unrelated to public/private information; like Donald said, the point is "to draw your attention to a card you're not paying attention to".
FWIW, I wish Scheme was a Reaction, for that same reason. In RL games, I forget to use its clean-up ability about half the time I've played it...