Other ISPs are businesses too, it all depends on the business model...
If your business model is primarily to provide connectivity to your customers, then settlement free peering is what you want because it improves the service as well as reducing costs.
If your primary business model is to sell peering then you don't want settlement free peering. Your customers paying for connectivity are effectively being used as a product which is sold to your peering customers.
The other providers all have open peering arrangements on SGIX, so they are prioritising the service being provided to their existing paying customers, and not trying to open a second revenue stream at the expense of existing customers.
This only works because most of the customers are unaware of this customer-hostile model, and the fact that it provides an inferior service to them. It also seems to be rather specific to singtel as large isps in other countries (including MUCH larger ones like comcast etc) do have settlement free peering.
