I don't think games are set to highest priority, and even if so, that's not the reason why the CPU runs at 100%. the CPU runs at 100% because it loops continiously and processes data (gfx, input, sounds, ...) if the 'game-loop' would wait for input from the OS, then you'd get some very bad results imho.
The roles are actually inverted, it's now the game that is taking a role of 'OS' and sometimes gives some time to the 'real OS' so that it can also process it's messages that are in the queue. Other programs just wait for inout before doing stuff, a games does stuff and does not wait for messages
