I read their chart way different than they do based on the title. The highest motorcycle ownership is California at over 800,000. Roughly 10% of all motorcycles are registered in California. Out of 50 states, that's way more than the #2 state.
Number of bikes per population isn't the same as highest motorcycle ownership by my definition.
Just my opinion.
Oh, I'd also argue that owning a motorcycle and riding a motorcycle are two different things. I would bet those #1, 2 and 3 states listed put on less miles than the average California rider does if, for no other reason than we have the weather for it. Also, we have the views for it. Nothing beats riding in California for spectacular views and the weather.
California has more motorcycles than the top 8 states combined!