\\\\My real question is why is blood in the urine a sign of infection?\\
It may not be....there are other conditions which may causes blood in the urine other than infections.
\\\\Might it be a tear or spot of some kind in the urethra/bladder? \\\
Yes it might.
///Other infections don't bleed, why this one? ///
Sooner or later in the process of infection, nearly all infections will affect the adjacent area and cause bleeding, how quickly and how obvious the bleeding will be, will depend upon the area affected.