Read the definition of "highway" under the Motor Vehicle Act. It covers more than just the "highways".
And be thankful you didn't get a ticket for Drive Without Due Care (Sec 144(1)(a) MVA), which is $368. That is what I usually give out with 24hr suspensions, along with any other violation tickets applicable. It is commonplace for police officers I work with and speak with to give out the Drive Without Due Care tickets along with the 24hr Suspension.
You were driving with what the Government of British Columbia deems to be an unacceptable amount of alcohol in your system, with your ability to drive a motor vehicle affected in such a way that you pose a risk to yourself and others on the road.