The San Diego Comi-Con Magic panel is but an hour or so away. This is a monumental moment for Magic players and designers and rumormongers, and R&D head Mark Rosewater is hard at work rehearsing his talk and prepping for questions, but nevertheless he's found time to keep up his Tumblr blog and talk about design there. Even when the questions are about fringe mechanics and theoretical color pie expansion.
When marobot5401 asked, "Green gets can't be countered on creatures and red gets it on spells. Green gets 'if an opponent makes you discard this, you can cast it for free' on creatures. Who gets it on spells?" he responded:
I assume it’s a tool of black’s enemies, so green and white.
This is, of course, all asked out of the context of the Return to Ravnica uncounterable cycle, which freely used blue, red, AND green to grant that ability to creature and noncreature spells. Uncounterability, as principally a blue hoser, goes to blue's enemies, so Mark extended that logic to discard proofing. The one notable noncreature spell to get this kind of ability is the outdated Guerrilla Tactics, so there is really no current precedent outside of creatures.
Free spells are very dangerous; this sort of free spell is thankfully mitigated by reactiveness and conditionality, so we'll likely see this kind of madness variant effect again. It would be cool indeed to see a board wipe or targeted removal spell like this, or the latter that becomes the former when forced to discard it a la Guerrilla Tactics.