How is this constitutional? From "The Californai tax that terrifies tech":
Entrepreneurs and investors in California can expect to receive a rude shock in the mail if they sold their company in the last four years. Not only did the state's Franchise Tax Board (FTB) eliminate a tax break on capital gains for small business owners and investors, it announced the tax would be reinstated retroactively. This means those who benefitted from the break can expect a bill for unpaid taxes, plus interest, stretching all the way back to 2008.