They don't.
The NW blackberries being talked about here aren't
Rubus Ursinus, but an escaped hybrid of the Himalayan Blackberry and the native blackberries, a huge franken-berry resulting from the same Oregon State University projects that resulted in the Marionberry and others.
These blackberries really will have 25-foot canes over an inch in diameter, tangled with old dried woody ones. The thorns are large and hooked. They are extremely aggressive and will envelop and destroy an entire house if left alone, slowly tear down trees, or completely dominate a field.
The best way to deal with a huge patch is with a bulldozer and a flamethrower.