To elaborate on the GP comment, the issue is not whether the brokerage reports, but how they do it. You generally compute capital gain income as (sale price) - (acquisition price). If you buy a stock on the market and sell it, brokers generally report both sale price and acquisition price to you and the IRS.
But for employee stock compensation, the broker can report sale price without acquisition price to the IRS. If you don't report the acquisition price yourself, the IRS will think that it's 0, and assume your income is much higher than the real value.
