When I added a resistor to a set of christmas lights where I cut off bulbs, it gets hot. Why does a shorter string of lights not need a resistor?
The strings are designed to use bulbs whose voltages sum to equal the supply voltage.  So a string that uses 20 bulbs for a 120V power source will use bulbs designed to operate at 6 volts.  And a string that uses 50 bulbs for a 120V source will use bulbs designed to operate at 2.4V.  When you are making hundreds of thousands (or millions) of strings you can have custom bulbs made for whatever voltage you wish. 

If you want to remove some of the bulbs and shorten the string, then you must compensate for the power the bulbs were using or run the risk of premature failure of the remaining bulbs which you are operating over-voltage.

A 2.4V bulb designed for a 50-bulb string is NOT "interchangeable" with a 6V bulb designed for a 20-bulb string. No matter how similar they may appear to the naked eye.