Sling took a shot at Comcast-owned NBC today, charging that — in contrast to ABC, CBS, and Fox — its owned stations won’t accept ads for the Dish Network-owned service’s low-priced streaming TV package. NBC-owned stations in San Diego, San Francisco and Washington, D.C. specifically rejected the ads, Sling says. The inaugural campaign, introduced last week, playfully likens cable companies to schoolyard bullies that force customers to agree to long-term contracts…