Why not just make it "All creatures lose horsemanship," like on Gravity Sphere or Mystic Decree?

Because then you can't later play "Destroy all creatures with horsemanship" "Destroy all players who use horemanship" and "Destroy the T+D team that thought horsemanship was a good idea (and exile the one that invented shadow)" ?

Dunno.

I didn't want to mention this, because I don't want to look like I keep coming down on you, Sorrow. But I don't think that works. In Magic, cannot always trumps can. If something says that something can be blocked, and can't be blocked, it can't be blocked. I know Flying has been inconsistent with this in the past, but, nowadays, flying's reminder text is:

"This creature can't be blocked except by creatures with flying or reach."

So reach is now built into flying. Unfortunately, nothing is built into Horsemanship. I suppose you could do something like "During your opponent's turn, creatures you control have horsemanship.", which, I admit, is really weird...

I'm horrible with the wording, so it's no problem.