"Redistribute under the terms of the GFDL."

Let me guess... Linux distributions are "stealing" Firefox? Oh, there isn't some holy war against that, so it's a good thing. Right.

Do Linux distributions take Firefox and laden it with advertisements?

Some do. Ubuntu replaces the default home page with an Ubuntu-specific search box that points to the highest bidder's search engine.

It is perfectly compliant with GPL/GFDL to take the product and fill it with ads. There is nothing wrong with doing that.

They'd certainly be within their rights if they did.

And it is perfectly within everyone else's rights to frown upon it.

The liberal licensing of Free software/content does not mean every use of those freedoms is a good and positive thing. And nobody is being inconsistent by treating it as a bad thing, while still recognizing it's within one's rights to do.