While I can't speak to the truth of the belief that people are actively moving to Seattle, I can say that there's little incentive for someone to follow basic property or drug laws in this city.
EDIT: To your point on how this differs from "Homeless people are lazy" I guess the idea is that we've created a situation where there are even more factors pushing homeless individuals to the streets and suburbs of Seattle, that Seattle has made homelessness more attractive in a sense.