if I understand this correctly they do not exploit prisoners for slave labor

You don't understand it correctly.

America as a whole exploits prison labor for its food production

Doesn't absolve McDonald's from participating.

The only ways to avoid prison labor in your food as an American person or business would be to either directly buy from farmers or exclusively eat imported

For the average customer, maybe, but McDonald's is a multi-billion multinational company with complete control over their supply system and the economic power to force suppliers to stop relying on slave labor.

Btw, this is about people in OTHER countries boycotting McDonald's. Countries where all slavery is illegal.

neither is feasible for a fast food chain

Bullshit. You're pretending that McDonald's is powerless when the literal opposite is the case.

if Europeans will add "stop using slave labor" to their list of demands from America then that's fair

Finally talking some sense.

but McDonald's isn't particularly guilty in this instance.

Aaand you immediately stopped again. As one of the most economically and culturally powerful corporations in the world, they have more opportunity to enact positive change and thus more of a DUTY to do so than most others.