I don't believe there's any realistic scenario where the Turkish military would attempt to seize the bombs, but if they did do something so insane the garrison would force them into fighting and killing hundreds of NATO personnel and capturing the rest. That sets in motion an inevitable response, and one that ensures any rouge military authority in Turkey would not last long.
There's similar logic to why the US maintains a force in South Korea. They aren't expected to solely defeat a full scale attack from the North, but can directly respond to smaller aggressions. If the north did overrun them, killing US troops, there would be no domestic political debate concerning whether to respond in the US.
To put it so bluntly as to be distasteful: they're there to die.
