Fifteen cable networks Thursday joined the anti-TV violence crusade by endorsing the same parental advisory notices on violent programming recently OK’d by ABC, CBS, NBC and Fox Broadcasting Co.The well-timed announcement undoubtedly will take some of the heat off the cable webs at Monday’s TV Violence Summit in Los Angeles. Broadcast network exex who grudgingly signed off on parental advisories earlier this month have since been pointing a finger at cable as the alleged “real culprit” in violent programming. Sen. Paul Simon (D-Ill.), who has spearheaded congressional efforts at limiting TV violence, said, “I welcome this move and commend the decisionmakers at the cable networks who have made this a priority.” The cable webs that agreed to the advisories include Arts & Entertainment, Comedy Central, the Disney Channel, the Family Channel, HBO, Lifetime, Nickelodeon, Nick-at-Nite, Showtime, TNT, USA Network, the Discovery Channel, the Learning Channel, MTV and the remainder of Turner Entertainment Networks. Networks such as C-SPAN and the Weather Channel were not asked to endorse the accord because “it would not have made sense” due to the nature of their programming, according to Showtime topper Winston Cox. The 15 cable webs also announced that each is being urged to develop a set of standards and practices by year-end.