Any of these platforms cost money to run and most are run by commercial enterprises.
There are infrastructure (transmitters, satellites, to control centres), wages, legislative fees amongst other things. Some is paid for by subscription, some by the channels. Left over is profit. If there is no subscription it is more likely to be paid for by the channel. Hence why it needs advertising (it also needs it to pay its staff and hopefully make a profit for the channel too).
Please note I know little of the workings of telly broadcasting, the above is just based on how the world works. I think the Internet has skewed people's perception with it apparently being free.
