Has it ever been specifically stated that Data is incapable of lying? or his programming prohibits lying to his superiors? His programming was created without superior officers in mind. I've always been under the impression he could lie just as easily as anyone else, and in this circumstance, he made a personal decision to withhold, based on the fact that this was a clearly personal situation, & Data had to grow as a person
It's not that Data is incapable
of lying, it's that doing so in this instance seems out of character.