Seems to me they were giving you the option of paying a fine, at which time your circumstance could be elevated to higher authorities who could determine whether or not the felony would be waived, thus enabling entry into Canada.
As it is, they let you turn around and return to the USA. However, if they hadn't accepted your circumstance as true, your vehicle and RV could have been seized. Those who transport firearms and drugs without the proper paperwork across the border can have their rigs seized under the law.
Your anger shouldn't be directed at the border agents who were doing their job, and who apparently gave you the best option available.