While Supercell will continue to operate independently, it’s hard to believe that a $1.5B investment from a 3rd party company would make them “more independent”. However, entering markets like Japan or China is extremely difficult without powerful local players, so Supercell probably took the right decision by letting Softbank and GungHo join them. Both Japanese companies are well positioned to help Supercell succeed in Japan, and they want to make sure that they will reap the benefits as well.
Japan is a market where customers easily pay a buck here and there, so freemium apps can be hugely successful. Given that Supercell already has the recipes to get people to buy in-app items, it’s not hard to see why Softbank and GungHo want to help them.