Why does single pole light switch have 3 wire cable when source is at the switch?
This circuit is being fed forward from this location.

The two blacks are constant hot, the red is switched hot.

If you follow the /3 when you get to the other end of it, the red is switched hot, the black is constant hot.  

Edit to add:

If this switch is feeding a ceiling box, this is a common configuration so that the light switch (red wire) only controls the light portion of a ceiling fan, allowing the fan itself to be controlled separately by the pull chains.  If there is currently just a light fixture, then they only used the Red switched hot wire, and capped the black.  The circuit could also, optionally, continue forward from the ceiling box using the black-constant hot, and white-neutral.