I don't see how there could be legal ramifications for not processing transactions? Witnesses are free to process whatever transactions they feel like, and @themarkymark has decided to run node software that ignores incoming transactions from Steemit. He's not really freezing Steemit's funds, he's just ignoring Steemit's requests to process transactions. If @themarkymark wanted to, he could also, say, only process transactions with a memo of "fjoqwfqa".