I've been thinking about this, trying to figure out why this is bothering me, and I've come down with this:
At age 17, you are old enough to know:
1. prostitution is illegal
2. you are legally too young for sex
3. engaging in underage prostitution will not only fuck up your life, but your johns
IF this girl willingly signed up at SeekingArrangements.com (and as far as we know, that's where they found these girls), then I have absolutely no sympathy for her. That doesn't absolve Matt from checking her ID, and absolutely doesn't absolve him if he knew she was underage.
IMHO sex work should be legalized (for adults). Then sites like SeekingArrangments.com could be forced to actually verify that everyone that uses their site is of legal age.