On the Uno, why does VIN require 7-12 volts and the USB only 5 volts?
USB's Vcc pin is assumed to be a stable, 5v supply, and is directly connected to the 5v bus on the Arduino board with no further regulation. Vin is assumed to be an unknown and unregulated voltage in the range of 7-12 volts. 

The 7v minimum is a requirement of the design of the on-board voltage regulator which needs the 2v "head room" to maintain its output at 5v. A 5v power supply could be designed with a larger input range, even including voltages less than 5v, but the simple design used on the board covers a range of sources at a lower cost than a higher-performance power supply.