Why is cascade delete not on by default for this relationship?

Question

I have a 1 to many relationship between LabelLineItem and DespatchPart.
I can't understand why cascade delete is off for this relationship.

There is no relationship defined in the context using the fluent API. There is no LabelLineItems navigation collection in DespatchPart, so there is no reference back to LabelLineItem.

public class LabelLineItem
{
    public int Id { get; set; }
    public int DespatchPartId { get; set; }
    public int LabelConfigId { get; set; }
    public string Content { get; set; }

    // Navigation
    public virtual LabelConfig LabelConfig { get; set; }
    public virtual DespatchPart DespatchPart { get; set; }
}

public class DespatchPart 
{
    public int Id { get; set; }
    public int DespatchId { get; set; }

    // Navigation
    public virtual Despatch Despatch { get; set; }

    //...
}

It's my understanding that one-to-many relationships default to cascade delete on. As demonstrated in the code sample above.

Whereas zero-or-one-to-many relationships default to cascade delete off as would be the case if either: - DespatchPartId was declared as int?, - The fluent API declared the relationship as optional i.e. DespatchPart.HasMany(p => p.LabelLineItems).WithOptional(i => i.DespatchPart).

But neither of these are the case which is why I'm confused.

FYI - I'm certain the cascade is off, because when I tested the cascade delete by removing a despatch part record (in SQLManagementStudio), I received an attempted FK violation in the LableLineItem table as I tried to remove a referenced DespatchPart record. This wouldn't have occurred if it the delete had cascaded to the LabelLineItem table.