They are just copying IPL. One of the most exciting part of IPL is a team can play at most 4 international players in a team. Which gives it an international feel. It makes it popular in the countries of the players participating as well becomes more thrilling to local users.
Television viewers are more important to the production but real viewers would be much more beneficial for the stadium management. And Australia can implement a safe way to make viewers present in the stadium.
The difference between IPL and the BBL is that the former league can sign almost any international player of it's choice, while a lot would rather skip the latter due to extremely low salary cap. After all, players are also professionals and money matters a lot for them. BBL salaries are lower than what they have in smaller leagues such as the BPL and the CPL. Last season, Sandeep Lamichchane even took a break from the BBL, so that he could play in the better paying BPL.