I believe it works something like this:

The exchanges provide the data, which in real time are considered proprietary. After 20 mins or so they are considered public domain and can be provided free of charge.

The brokers want you to have the real time quotes to make you trade more. Therefor they provide the RT to you, although it may cost them money to do so. However, by trading you pay them a fee. So one way or another, you pay for the real time quotes the broker provides you.