Right. That's what I thought.
Apparently Joe Thornton didn't think so either, as he needed some extra convincing from Toews to accept the challenge. Toews was upset at the Sharks captain for going down rather easily after a shove to draw a boarding penalty. You can even see Thornton sneak a look at the referee.
Incensed, Toews went after Thornton, but had to shove him seven times before Big Joe agreed to drop the gloves. Thornton clearly was prepared to claim innocence and get off scot-free for buying the call, but Toews was too frustrated to let him off the hook. Apparently these two have a history, having been involved in a "skirmish" last year. Toews' last fight was with David Backes in 2010, and Thornton last tussle was with Drew Doughty last season.
H/T to HockeyFights for the video.