Income tax on issued shares

If a business partner of a start-up is compensated with shares in accordance with a vesting schedule, apparently that partner will owe tax on those shares in the year they are issued. So actually, they would be out of pocket for the work they contribute (until those shares later pay out dividends or they sell those shares). Is this correct? This is what I'm reading and it makes no sense to me. Why is tax owing for receiving shares? If I go and buy shares in a public company I don't owe tax. Completely bizzare. They would essentially have to sell some of those shares to cover their tax burden, but since this is a start-up there is no open market for that.