Elipsepearl - this is a rule currently. You may not up on the current rules as your byline says that you used to be a flight attendant. I had a friend look these up for me from the flight attendant manual:

Sec 3.730 page 4 "No lap children allowed in row 30 if all seats in row occupied." (737-800 only)

Sec 6.400 page 8 "To ensure enough oxygen masks are available, only one lap child per row (including both sides of aisle) is allowed if all seats in a row are occupied."