Same reason Wings fans all during the mid 90's and in to the end of the division believed the Blues were thugs.
When a fanbase typically sees ice-dancers that fanbase thinks everyone should play no-touch, offensive-only hockey.
Therefore a grittier player like Backes looks like Marty McSorley on a PCP-binge because he gets a bit physical with their players; to say nothing of the Blues' consummate troll, Barret Jackman.
I have to hear about the Blues goonery (and Boston's and LA's) on a regular basis from people who were unaware hockey existed until 2009...thus I am rather well-versed in this particular realm.