Rams vs Cardinals a sucker bet?

[StLouisMike]

There is a radio show where I live that does weekly pics and when the STL vs ARI came up they said it was a sucker bet yet they never bothered to tell why. For the experienced bettors out there can you please tell me why? I do not bet on sports but, it interests me and I would eventually like to start. St. Louis was a 9.5 favorite and Arizona won the game by ten. To me it seemed like an easy pick for Arizona as the Rams barely beat them in Phoenix a few weeks earlier. Thanks for all your help

Mike