I had to look this up again, as I recalled something from my HS Chemistry days.
The Mallard Reaction: amino acids and reducing sugars combine with heat to produce hundreds of smell and flavors.
Without additional sugars, the naturally occurring sugars will combine with the amino acids (I recall these might be part of what makes up fat) forming the bark.
I don't think the addition of sugar in the rub adds as much "candy sweet" taste as it does the more complex salty/acidic/sweet flavors of the mallard reaction (think bacon!!!!)
2 WSM 22
La Caja China #2
Weber 22 Kettle
Brinkman Vertical Charcoal (out on loan)
Weber Q 200
Weber Summit Series Gasser
super sneaky thermapen disguised as a protemp