Do I understand this correctly -- if there is a MUP that parallels the busy road why not use that instead? Maybe that is why the motorist was upset?
Personally, i don't understand why cyclists who ***** about not having bicycle friendly roads don't use the paths when they are provided for the safety of the cyclist.
Sometimes separated paths are provided with the intention of making cyclists safer, but in reality make cyclists less safe by putting us in places where we won't be seen-and-noticed by motorists when we approach driveways or intersections. Sometimes, using less-safe facilities is even mandated by law.
Weren't you legally in the wrong then, if the city ordinace tells you to be on the MUP?
It looks to me like N_C was violating a law by using the road, but without seeing the location ourselves it's difficult to judge whether the MUP was reasonable for N_C's use. If in fact N_C was violating a law that's ill-conceived and not enforced, we probably don't care that it was violated.
Cyclists can almost always safely use separated paths that parallel roads. But safe use of those paths often requires pedestrian-like behavior at driveways and intersections and that could lower your average speed by 1% or 50%, depending on how many times you have to stop or slow to a walking pace for intersections.