They are playing base ball amateur championships here. "Amateur championships" is good.
Does putting "amateur championships" in quotes in 1874 mean the same thing it does in 2013? I'm thinking it does.
We have enough evidence to suggest that St. Louis players were getting compensated in some form by the late 1860s and I've found a few subtle references like this in various papers. The guys who were playing for top clubs like the Empires, Unions or Reds were getting paid or getting jobs at the fire department or with some construction firm owned by a fellow club member. Tom Oran wasn't jumping from club to club to club because he was fickle. He jumped from the Unions to the Empires for a job with the St. Louis Fire Department and, I assume, he jumped from the Empires to the Reds for a better share of the gate.
They were amateurs only in the 19th century baseball sense of the word.