Well, looking at this further I see that this would also work:
Simplify[Sqrt[(a b c + d e f)^2], (a b c + d e f) >= 0]
So zero needs to be included in the domain of this case.
But when the expression is shorter there is no such need:
Simplify[Sqrt[(a b + d e)^2], (a b + d e) > 0]
This is puzzling