Amusingly enough, I never check the magic TCG reddit, and I checked it anyway today (before Alex posted the link!) for totally unrelated reasons and saw the hubub. WTF indeed. Horrible, horrible decision.
Reiska: You're giving WotC too much credit. It's just a plain mistake, with the largest reason being what Alex already described - this decision will either entirely kill Legacy, or it will perversely incentivize TRUE counterfeits - the kind that every store owner who's invested in card stock hates - rather than harmless proxies + actual stuff bought from the store. Legacy & Vintage will eventually die? Fine. Let them die. Just don't upset the status quo, and the status quo is helping them die slower. Obviously Wizards can't openly sanction events that allow proxy cards, and that's fine, but why not just ignore them? Even if hypothetically WotC makes no money off them - a very false claim - it's not particularly harming WotC either. It's a baffling & bizarre decision to threaten stores that are basically holding hobbyist events for the highly-committed that encourage them to spend thousands of dollars on the game, some of which will flow to the stores which will then be incentivized to hype up Magic more.
Anyway, I highly doubt it's because Wizards has some secret master plan to force Legacy players to play more Standard or something, or Legacy will somehow ruin them. It's either some strange legal advice they got (this would be the favorable-to-WotC-but-plausible tack, IMHO) or Hanlon's razor applies as usual here (more likely IMO).