"It’s not as if there’s any ambiguity about the correct side of the road to drive on."

Actually, it's not anywhere close to as clear as you indicate in Ontario.  You can drive to the left of centre unless one of the exceptions applies.

Driving to left of centre prohibited under certain conditions
149 (1) No vehicle shall be driven or operated to the left of the centre of a roadway designed for one or more lines of traffic in each direction,

(a) when approaching the crest of a grade or upon a curve in the roadway or within 30 metres of a bridge, viaduct or tunnel where the driver’s view is obstructed within that distance so as to create a potential hazard in the event another vehicle might approach from the opposite direction; or

(b) when approaching within 30 metres of a level railway crossing.  R.S.O. 1990, c. H.8, s. 149 (1).