It is interesting how they make the exact opposite decision in that episode as Picard did with Jono. This implies to me that the decision in that episode arose out of 'Human guilt' rather than moral imperative.
Rugal is a minor so his feelings wouldn't have any legal standing, and besides his feelings were the result of psychological manipulation. He would probably have a better life growing up on Cardassia than Bajor, because at least that way he can grow up with a sense of who he is and where he came from through a lens that isn't 'All Cardies are evil conquerers'. A normal life for him on Bajor was out of the question whereas a normal life on Cardassia is possible after an adjustment period.
In the long run, although his adoptive parents clearly loved him strongly, you simply can't justify forcing him to be a lifelong pariah.