How come it's ALWAYS a serious breach when the other guys screw up but only political posturing by the opposition when your guys screw up? How does that work exactly and why is it done so often?

That's politics as a whole, I expect as much from politicians.

But the thing is, if people held the politicians they support to a higher standard of conduct than those with whom they disagree, this wouldn't be an issue.

People are swayed by anyway the wind blows these days.