A business owner or CEO in the 250k bracket that has his taxes raised is not going to tell his company to hire fewer people.
Partnerships are pass-through entities, meaning the owner and the company are the same for tax purposes. The same is true for S-corps and LLCs taxed as S-Corps (though LLCs can elect to be taxed as C-corps, but it only makes sense to do so if your company is expected to re-invest profits in the company).
Additionally the business owner will anticipate his own reduced income and distribute a larger share of the company's profits to himself to compensate, reducing the cash pool available to hire new employees.
It's a valid point. It's less valid than taxing the fuck out of the top end and giving tons of free money to the bottom. The bottom is going to spend it buying the top half's products anyway, making them richer ten minutes later than they would have been if we reduced their taxes. Tax reductions don't make people rich, sales do.