They let people believe that streaming is cheap, but it is not. A server can send streams to many people at the same time, but not so many as it seems and sever up time is a cost, in terms of energy and in terms of sysadmin time. Maintenance of the network is also expensive, especially in the US where most of the people live in low density neighbourhoods.

The dirty cheap subscriptions were meant to attract new customers, the service was heavily subsidized. The companies looked profitable just because other companies bought more ad space than necessary. Overadvertising is the preferred method to give stealth subsidies, but it is a cost for the other businesses of the network. After a while they have to shift those costs to the customers.