But you have to understand why the church as allowed for economy in receving converts, and under what conditions determined the form of recieving certain heretics/schismatics .
The short answer would be because of pastoral reasons. You could also argue political if you pushed hard enough. Basically the Church did what ever was most beneficial to Her and to the souls she is/was trying to save. If it would help bring in large numbers in to the Church, or to help prevent violence against Christians, the Church exercised economy. Provided the Apostolic form of baptism was maintain in the heretical/schismatic group.
In the case of the Russian practise, prior to the 17th century council the practise was to baptize Latins. But, according to Met. Anthony, when large portions of land were aquired by the Tsar that contained many Uniates economy was used to facilitate receiving large numbers of converts. It wasn't mentioned, but I'm sure they still preserved Apostolic Baptism (thrine immersion and emmersion).
A reason for only chrismatind Arians was 1) because they preserved the Apostolic Baptism and 2) because it became confusing determing which Arian converts had Orthodox Baptism (those baptized Orthodox, went to the Arian, and then repented) and which had the polluted Arian 'baptism'.
But I can never stress enough, despite the apparent contradiction canons allowing economy give, the Church has never, and will never recognize any amount of validity in any heretical/schismatic baptism. There is only one baptism, every thing else is a pollution. Thusly, baptizing converts doesnot violate any canon (or the Creed) against rebaptizing. If local churches wish to receive converts from heretical/schismatic 'churches' who maintain an Apostolic Baptism (3 immersion) for the sake of saving souls, or just for the overall benefit of the Church, so be it. The rule still is every one is baptized.
