Old way of seeing prostitutes in the criminal justice system = criminals.
New way of seeing prostitutes in the criminal justice system = victims.
I feel that the movement of patronizing a prostitute from your typical slap-on-the-hand misdemeanor to something that, even though still a misdemeanor, has the "sound and feel" of something more deeply disturbing to society as a whole is probably the result of more progressive attitudes toward sex work and law enforcement. Prostitute, in and of itself, is a punitive term. There's a lot of judgment. Through shows like Dateline's "To Catch a Predator" and various/sundry other types of media concentration on human trafficking, sex crimes, and pedophilia, the term "sexual exploitation" has become better known in the cultural vernacular.
Given the high instance of trafficking, drug dependency, addiction, and the inherent social problems therein, attempting to stem demand for the services of sex workers is the new, in vogue, way of dealing the world's oldest profession. Granted, there are some women who are happy hookers. They participate willingly in the activities and for them, it's a job like you and I go to work at an office building everyday (with our clothes on). Unfortunately, the odds are favorable that your local sex worker is not truly in the business for herself. She's desperate for one thing or another: drugs, money, companionship, attention - whatever. These are the girls that are picked up for prostitution over and over again. The "working girls" tend to be able to avoid that because they have access to more private accommodations, a more discrete clientele, or the ability to work largely without a pimp/agency/service, etc. The minute a woman has to hit the streets, the unscrupulous characters come out of the woodwork - if they didn't BEFORE she ended up there.
I can see where it would be a controversial topic to move prostitution arrests into "sexual exploitation" because some people feel like prostitution is a victimless crime. There aren't a lot of Johns out there that would feel like they contribute to human trafficking - I don't think this microeconomic correlation is the first thing on their minds (and thus, they don't want to be criminalized for their behavior).
I thought the same thing at first, Lisa. The general category is Sexual Exploitation, which usually refers to children. But after looking at the number of the law he broke, it isn't about children. Apparently Seattle includes his charge in that category.
ETA: Seattle changed "patronizing a prostitute" to "sexual exploitation" which apparently is quite controversial.
http://reason.com/blog/2015/01/14/seattle-change-to-prostitution-code