Yes, because Facebook doesn't exist only in Germany or only in the US.
If I, as a US citizen, want to deny the holocaust on Facebook, FB then has two choices - Remove the offending comment entirely, or at least block it from viewers in Germany. Either of those infringe on my right to express whatever brand of bigotry I may subscribe to despite living in an entirely different country that doesn't feel the need to outlaw critical thinking. I might not get arrested for it, but I would have had my voice silenced as a result of Germany's stupidity.
FWIW, I don't count as a holocaust denier. I arrived at that conclusion through rational consideration of the evidence, however, not because my government told me what to think - And in fact, the latter would make me less likely to believe it; any time the government really wants you to believe something, that raises the bar for the actual evidence a hell of a lot higher.