It is perfectly legal to watch TV online without paying. Many TV networks encourage viewers to watch their shows via the network's own website. PBS, ABC, NBC, Comedy Central and BBC America are among the TV networks that actively promote online viewing, as of 2016.
TV networks make money by selling advertising space to retailers and other parties in exchange for airing ads during commercial breaks. This process occurs with both traditional TV viewing and online viewing.
When watching online, viewers see roughly 10 to 15 minutes of a show, and then there is a mandatory pause while a commercial plays. Once the commercial airs, the show resumes. This is what makes it possible for networks to allow viewers to watch shows online without paying.
Many network ask users to create accounts on their sites before they can watch videos, but these accounts are free of charge.