Because otherwise, even if bitcoin-denominated, the employer still needs to do compute the fiat value for determining the income tax withholding, and unemployment tax, and Social security, and such.

Oh,... I just realized something. Agreeing to pay a Bitcoin-denominated salary can really make things complicated for doing this with a W-2 employee. In addition to paying the salary the employer also has an employer paid-taxes, for social security and medicare, as well as unemployment taxes. So if the BTC/USD exchange rate rises, the employer's tax payment will rise as well. The employer not only needs to hedge for the salary but for the resultant employer-paid taxes as well!!