The word "hijacking" in this scenario would only be applicable if the domain was still registered and active and he forcefully took the domain away. That is not the case. The fact OP was able to register it quickly and easily indicates it was unused and to call this "hijhacking" would imply permanent ownership of domains even after previous owners knowingly let the registration lapse.

The legality of hosting a tracker isn't obvious, and as pointed out elsewhere the nuance is less about concrete legality and more about having the resources to deal with lawyers harassing you with lawsuits.