Why led turning on when switch is low in pic starter kit?
You should refer to the User Guide. It says that

Push button switches provide the following functionality:
• SW1: Active-low switch connected to RD6
• SW2: Active-low switch connected to RD7
• SW3: Active-low switch connected to RD13

also

The switches do not have any debounce circuitry and require the use of internal pull-up
resistors; this allows you to investigate software debounce techniques.

So I think this answers your question of why led is turning on when switch(RD7) is low