The point of the piece isn't to literally say cowboys don't read poetry. It's a play on masculine stereotypes. In this case the stereotype is that things like poetry which are usually deeply emotional pieces of art is a feminine thing, or as stated in the poem a "sissfied" pass time. The writer even shows the cowboy characters in the end being intrigued by the poem and liking it but brushing it off and acting like he meant no such thing. He acts like that because he thinks the act of reading poetry is too feminine for a cowboy.
