Tick Tac Toe

I was halfway home - I was half insane,
And every shop window I looked in just looked the same
--Style Council

Many lament that elimination of the 'uptick rule', which permitted selling a stock short only if the previous trade price was lower, has exacerbated downside volatility in the markets.

This bids an obvious question. If it is not ok to short stocks on a downtick, then why is it ok to buy stocks on an uptick?