Bitcoin is not linked to your Bank Account.
You might be confusing the idea that some people chose to open up an account with a regulated online exchange, and chose to link that exchange to their bank account.
There is nothing illegal about Bitcoin. It is recognized as Decentralized Virtual Currency and/or an asset by most governments, and while they may not like the idea, and warn against it's potential volatility, there is nothing illegal about it.
The IRS cares about Bitcoin in the same respect that it cares about the exchange of Apples & Oranges or anything of value.....even illegal valuable items like Marijuana. That's is to say, they want you to report it on your tax return. Legally speaking, you must report ALL your profits from your Marijuana Growing Operation, or risk a Tax Evasion case (think Al Capone and his Tax Evasion woes)
Likewise, if you earn a profit in Bitcoin, you are expected to report it to the IRS.
A brief snippet from the IRS on Bitcoin found here > https://www.irs.gov/uac/Newsroom/IRS...rency-Guidance