I haven't cured bacon but I've read about it a lot. I've cured sausage
at home, and the principle is the same. This is my take on the sugar
issue with regard to cures:

Sugar does two things. It is an anti-bacterial as everyone knows. It
controls bacterial growth during the cure, to some extent.

Most importantly, though, it keeps a dry cured product from getting too
hard and chewy. That is why it is used. Once the cure is done, the sugar
and salt is washed off and only a bit remains in the meat. It is not
used on fish (smoked salmon) necessarily, because fish is softer meat
and not cured with the intent of sitting in a cooler for weeks and
weeks. Without sugar, pork for instance cured becomes very hard and
tough. PEople who buy bacon want it to be pliable, not like a rock hard
country ham.

--Richard