The rules define a good return as. The relevant section is 25a. It very clearly states that a ball hit into the net is still a "good return", provided it hits the ground within the correct court.
25. A GOOD RETURN
It is a good return if:
a. The ball touches the net, net posts/singles sticks, cord or metal cable, strap or band, provided that it passes over any of them and hits the ground within the correct court; except as provided in Rule 2 and 24 (d); or
b. After the ball in play has hit the ground within the correct court and has spun or been blown back over the net, the player reaches over the net and plays the ball into the correct court, provided that the player does not break Rule 24; or
c. The ball is returned outside the net posts, either above or below the level of the top of the net, even though it touches the net posts, provided that it hits the ground in the correct court; except as provided in Rules 2 and 24 (d); or
d. The ball passes under the net cord between the singles stick and the adjacent net post without touching either net, net cord or net post and hits the ground in the correct court, or
e. The player’s racket passes over the net after hitting the ball on the player’s own side of the net and the ball hits the ground in the correct court; or
f. The player hits the ball in play, which hits another ball lying in the correct court.