This is a very typical Hong Kong way of pricing: when you get discounted hotel buffet offers, they will charge you the discount price plus 10% service charge from the FULL price of the buffet. I don't know if it's legal and this has been on for as long as I remembered. In your case I wouldn't see it's discriminating infants but a way to nickle and dime people.

Hong Kong Airlines is owned by Hainan Airlines and it's kind of a full service low fare airlines - we have 3 kids and we have been purchasing infant tickets from various airlines and never encounter problem like this (although they varied from 10% of the all in fare or 10% of base fare plus tax - so we paid around $50-$120 per infant ticket). We always called the airline we want to book with to confirm their infant ticket policy - if you have done that the situation would never have happened.