I've connected this H bridge driver chip to my arduino UNO as in the diagram
http://www.hobbytronics.co.uk/motor-control/h-bridge-driver-sn754410
But oddly the logic levels are reversed:
Sending the arduino pin HIGH connected to "motor 1 forward" should turn on the motor but it doesn't it turns it off.
Sending that same pin LOW turns the motor on.
Seems this is actually how its supposed to be from reading the arduino code on this page:
Live and learn.
According to the datasheet, it should be operating the way you expected it to originally.
The only way it wouldn't is if you had both pins set high to begin with and pulled one line low. But it would move in the opposite direction than you expected in that case.
If not that, then the datasheet is wrong.