I think that is an error in the article. I live in Boise. The law allows ticketing if there IS room in shelters. People would not be ticketed if the shelters are full.
Not saying it's a good law. But the article is wrong.
Edit: Here is the relevant section of the actual code.
> Enforcement: Law enforcement officers shall not enforce this camping section when the individual is on public property and there is no available overnight shelter