User blog comment:Nearthel/Bellomarus, the Destroyer of Worlds (Background for my next custom champion)/@comment-3391671-20160421000501/@comment-3391671-20160422025313

As per his backstory, Aurelion Sol did come to Runeterra through it. But it elaborates more that he came to Runeterra to connect "with one of the few races he deemed worthy," the Targonians. But when he got bored and attempted to leave Runeterra to go back to star-forging, a gift the Targonians gave him drew him back to Runeterra. "{...} for each attack against his newfound masters, one of his stars vanished forever from the firmament."

The last part is what binds Aurelion Sol to Runeterra as his precious stars are in danger if he doesn't work in servitude. Of course, he has voiced an interest in "(dragging) the very stars from the sky, if he must, in order to regain his freedom." Otherwise though, if another being is already destroying his stars willingly, then he would care less already if any more stars are destroyed when braking free from the Targonian chains.