Not all pet stores are bad, so I'm not 100% against pet stores, but you will likely severely overpay for a snake that probably isn't the healthiest or best quality. Also, I don't like the idea supporting shady/ill informed/greedy pet stores with my money just so they can order more animals and neglect them. That is most likely the general consensus of why most folks here don't approve of pet stores. Again, not all pet stores are bad. There are good ones, so chose carefully.
Also, I highly recommend you feed your ball rats instead of mice. There are a million threads on this forum regarding this subject.