Why do American cable systems pay retransmission royalties for distant signals?
The short answer is because copyright laws in many countries, including the United States, require them to do so. The rationale is that a cable system that retransmits a distant signal benefits from doing this. The programs on that distant signal expand and enrich the selection of programming that the cable system delivers to its paying subscribers. The other side of the coin is that the copyright owners of the programs deserve compensation for use of their works.
