Pretty sure Watto had to sell her to help cover gambling debts from anakin winning. I could be wrong, but it would have been fairly simple for the Jedi to send somebody out to take care of it.
But then the Jedi are all brainwashed slaves anyhow.
The Jedi are not supposed to interfere, something they do frequently regardless. Qui-Gon was already known to bend the rules but he wasn't so impulsive as to kill a slave owner that isn't attacking anyone under his care.
The Jedi Order was hypocritical and paranoid. Their hubris and arrogance led to their eventual downfall.