> the beef portion of the hamburger patty is "100% PURE ALL-AMERICAN BEEF"

But the claim isn't about "the beef portion". If what you say is true it would give a competitor the chance to say "McD's burgers are 100% unhealthy fat" because the unhealthy fat portion is 100% unhealthy fat.

> It could be 99% filler and not be a lie.

I doubt it. Not sure about the US but in Europe there are laws that tell you how much or how little of something you can have before you advertise that thing. For example you can't say "now with less sugar" if you reduced the quantity of sugar by 0.001g. And you can't call it "olive oil" if it has 2% olive oil.