It's actually state law. I'm the general manager of several LLCs and I have to maintain separate accounts for the funds for each one. I addition, I have several personal accounts that I use to manage my own finances. The only difference between classical banking and Bitcoin is that tying a Bitcoin address to a tax ID or SSN isn't required like it is for a bank account.
If the IRS doesn't like that, they can go fuck themselves. Business licensing and corporate charters are powers of individual states, not the federal government.