Well, in Europe the Venn diagram of the two has enough overlap that people assume they are closely related. That, and things like humane treatment of livestock are expected to be better with certified organic produce.
IIRC, some organic certifications like Demeter and BioLand also have sustainable production as part of their requirements to get their seal. And since they were there before the EU-seal was codified, people just assume organic farmers are also performing sustainable agriculture.
