Odd bypassed resistor network in the representative circuit of an MC7805 - what does it do?

These resistors are part of the voltage feedback network. Depending on how many of these resistors are in series (meaning: shorted or not) you get a different feedback network.

The feedback network sets the output voltage. So they use this to make the different 7805, 7806, 7812 etc... regulators from the same silicon design.

Basically they're all "born" as 5 V regulators. Then by "(laser ?) zapping" the shorts across the resistors they can be made into regulators for different output voltages.

My guess would be that the resistors are used for the configuration of the device.

Integrated circuits are made in several process steps the last steps are where the metal interconnections are fabricated.

Voltage regulators are made for different voltage levels and the actual value of the feedback network and thereby the output could be changed as one of the final process steps by changing one or two masks.

If all resistors are short-circuited you would have a generic voltage regulator with a certain divider ratio you would get one of the standard voltages.