If this is true (at least for non pro customers) the reason behind should be to hardly reduce bandwidth / traffic costs for the service. Soundcloud was mainly getting known as a free Hoster of large audio files. But SC has to go shopping for IP bandwidthes too - as any Internet-User.
I dont know the numbers of their company in detail, but it should be hard to generate money with such a service today.
But it could be a feature too if they offer a player / streaming solution which really adapts the requested streams to a available bandwidth and processing power for each end user. There still are enough users on this globe and users in different usage scenarios which can’t exceed more then i.e. 56k - even on a theoretical 20 Mbit/s mobile link. This is something a typical web hosting is not capable of.
The Internet costs money - each transferred byte from one hop to another costs money and energy and the prices/costs are still higher then many of end users believe which only “knows” their mix-calculated “Flatrate” products typically.