That's a pretty solid ROI.
Apple has huge profits and a cash pile despite a) having lots of competition, and b) returning profits to investors via dividends and share buybacks. Perhaps there are more options than the two you present?
Of course, Japanese capital isn't the only capital that chases stupid stuff, especially not right now. We're all Japan these days. :)
They're an insurance company which monetizes through operating businesses rather than the traditional model of bancassurance.
http://www.berkshirehathawayhs.com/
They also own 100% of a heck of a lot of subsidiaries. They're not just minority owners in a bunch of companies like Coca-Cola.
https://en.wikipedia.org/wiki/List_of_assets_owned_by_Berksh...
If you're good at writing insurance it's essentially a great way of getting interest-free loans which they then spin into a huge pile of low-risk, cash-generating businesses (e.g. Duracell, Fruit of the Loom, See's Candies, to pick three Berkshire brands...).
The second alternative doesn't make sense. You can't say a company has lots of profits because they don't return it to investors.
Earnings (profits) is basically Revenue - Expenses. Whether the company chooses to retain earnings to reinvest in itself, or to return profits to investors (by paying a dividend or repurchasing stock) doesn't change the value of earnings.
In any case Softbank does pay dividends to its investors.
Japan's telecom used to be rather non-competitive, with 3 major carriers dominating the market, mandating locked phones and very likely colluding on pricing and fees. But since we got guaranteed number portability and MVNOs things have been better (I've just switched from Softbank to a MVNO, going from paying $70 monthly to about $25).
Softbank's earnings come not only from domestic telecom but also from its stakes in Yahoo Japan (search/advertising, auctions, loyalty points etc) and Alibaba.