"The rule that they're referring to doesn't apply if someone is employed as a writer," he said.
The guild's muted reaction to Leno's on-air admission was a stark contrast to WGA East's fierce public scolding of Ellen DeGeneres when she resumed production of her daytime show shortly after the strike began.
The WGA shouldn't have been surprised by Leno's decision. In a meeting Monday with Verrone and the show's writers, the comedian explained his intention to pen his own material, according to an NBC executive.
"He said, 'I'm going to do a monologue, I'm going to write it,' " the executive said. "They were well aware he was doing it."
Sacharow said guild officials did not give Leno permission to write.
"I understand there was a discussion, but there was nothing said there that would lead Jay to believe that he could do anything to violate a strike rule," he said. "If that's the way it was interpreted, then it was a misunderstanding."
The union's next move will be followed closely by its members, some of whom are divided about how the WGA has handled the decision by the late-night hosts to go back on the air.
Until they decided to resume production this month, all the hosts ensured that their nonwriting crews continued to be paid. Leno, fellow NBC host Conan O'Brien and ABC's Jimmy Kimmel all helped cover the salaries out of their own pockets, moves that were warmly touted by their striking writers.
So the union's decision to picket outside the studios where Leno and O'Brien are taping their shows this week was not universally embraced.
But if the guild gives Leno a pass for writing, it runs the risk of alienating its own members. On Thursday, many posted angry messages on strike-related websites.
"What the guild has allowed Leno and Conan to get away with is disgraceful," read one comment on the writer blog United Hollywood. "If we do not ramp up our attacks on these scabs, we can fully expect show runners and feature writers to follow Leno and friends back to work."
Times staff writer Richard Verrier in Los Angeles contributed to this report.