It only affects their FG% when they MAKE the shot. When a player is fouled while shooting and the shot is made, then he will be credited with 1 field goal made and 1 field goal attempt. On the other hand, when he is fouled while shooting and in the process missed the shot, then he will not be credited with a field goal attempt.
BTW, MJ's FG% was pretty high. For his career, he hit 0.505 of his field goal attempts, not counting his shots when he played for Washington.
And to BillH, the part of the NBA Rules you refer to (Rule No. 4 - Section XI) is written a folllows:
"A field goal attempt is a player's attempt to shoot the ball into his basket for a field goal. The attempt starts when the player begins the motion which habitually precedes the actual shot. It continues until the shooting effort ceases and he returns a normal floor position. The term is also used to include movement of the ball in flight until it has become dead or ha sbeen touched by a player."
Nowhere in that rule can you find the answer to the question. What you need to do is watch an NBA game and keep track of the field goal attempts of someone who gets fouled often, say Iverson or Pierce, and check the boxscores afterwards.