The disappearance of Nickelodeon, MTV and Comedy Central from the TV sets of 20 million American homes on Wednesday marked a line in the sand drawn by one of the biggest pay-TV distributors in a dispute over programming fees with a major entertainment company.

What might once have been a run-of-the-mill spat has taken on heightened importance because it occurs at a pivotal time in the TV industry. Low-price or free online video outlets like Netflix Inc., Amazon.com Inc. and Google Inc.'s YouTube are emerging as serious competitors...