I wouldn't call it the "signal source" only because it is more than that.
you would not be able to receive anything other than their programming with a "satellite" radio, there is the antitrust.
if you were able to get another providers signal then it would not be an issue.
it's kind of like TV, you can get multiple satellite types, cable, and plain over the air, but a choice of what to pay for or not to pay for.
personally though I'd say time warner has more of a monopoly on "pay" TV and a bigger issue than satellite radio, but that will never change I have come to believe.
anyway, back to the topic at hand, yes our society has gotten "used" to paying for content, and it is a new type of situation with "pay" radio (and personally I can't see paying for radio ever) but I have to say, there needs to be some sort of competition in the market, or it might as well be like the BBC