Why will contaminating bacteria make wine sour while pure yeast will not?

The bacteria can make the wine sour but the yeast will not. Yeast can grow in the presence of air or in the absence of air. But alcohol will only be made in the absence of air known as Fermentation.

It does not take place in an open container. Certain contamination with bacteria can convert pyruvate to acetic acid instead of alcohol which gives sour taste to the alcohol.

So adding yeast does not makes the alcohol taste sour but the contamination of bacteria can.