Because only right-leaning organizations are valid sources.

No, because ideally you'd have an ideologically neutral source, specially for politically charged topics. But such sources are essentially nonexistent, so we're left trying to subtract out the biases from wikipedia articles.

Don't get me wrong, wikipedia has value; it's just not a trustworthy source for certain topics. At the very least you should take a peek at the "history" tab to see alternative viewpoints.

I have always found it amusing that people who use this slogan tend to be deeply concerned about bias from the other side but see no problem with such a glib statement.

Because that statement is meant as sarcasm, highlighting the absurdity of certain claims, and not a literal statement.

That's the thing though, I never hear that statement from conservatives or non-liberals - they never seem to utter it, even in a sarcastic or mocking tone.

I only ever hear it from what seem to be sincere liberals/progressives, and often in the context of dismissing claims of liberal bias. In fact, that seems to be exactly how it was used in this very comment chain.

Wikipdeia is double plus ungood for containing thought crimes against The Party