Public housing policy is meant to provide basic housing for citizens. Let's take the example of a newly wed couple. They buy a BTO flat with grants and subsidies. Basically they started out humble and simple. Years later, when they have accumulated more money, they buy a condo, and earn their rental income. This is allowed, because what they have done fits the purpose of the public housing policy. The policy helped them when they needed it, and later on in life when they accumulated more wealth and bought private, they are allowed to because it is not against the principles of the public policy.
In your case however, you decided to buy a condo. Why should you be allowed to use the public housing policy to buy a HDB for rental income? You obviously are well off (if not wealthy) enough to afford and buy private. There is no reason to allow you to use the public housing policy since you did not need it in the first place. Instead of staying with your parents until you were 35 and starting out humble and simple by buying HBD, you decided to show that you could afford a condo. Good for you if you made this choice. But no HDB for you. (unless you sell the condo).
I have to admit that I absolutely love this pap policy.