Most of the 1908 wallabies who won gold at the London Olympics defected to league the year after.
The ARU thought RL wouldn't last when it started in 1908, but they did their best to make life hard for RL.
The Wallabies defections in 1909 was the catalyst for the hatred that still exists today.
They thought it immoral that players get paid instead of the game administrators. By keeping the players unpaid, they kept the games integrity pure & intact, in their view.
Many sports were amateur, for many years. Not only rugby union. Tennis for example, only amateurs could play in the Davis Cup, Wimbledon, etc, until 1968 I think it was. Athletics, swimming, all the Olympic sports were also amateur.
The antipathy between the two rugby codes has more to do with class and sectarian rivalries than anything else (in Australia at least). Professionals in all sports were thought to be inferior to the "gentlemen" so called, who played the sport for love. Look up the root of the word "amateur".
English cricket had a Gentlemen vs Players match every season for many years. No prizes for guessing who the "gentlemen" were.