In Seattle, there are a lot of people living in tents, cars, and temporarily-parked RVs. They all get lumped under "homeless" because they don't have permanent addresses or access to municipal services.
> The only remaining argument is basically "this encourages already homeless people to move to Seattle", which I can see being a problem, but of a different kind.
It's really hard to get good data on this, but, yes, there is a widely shared belief that Seattle gets more than it's "fair share" of the country's homeless epidemic because we have a lot of compassionate people, services, and, increasingly, lax law enforcement. Talk to any local and they'll tell you anecdotes about other counties or even states buying people one-way bus tickets to Seattle so that it's not their problem any more. It's hard to tell how much of that is urban legend.