It makes sense if you see sex workers as victims, which is what the bill does.
The government can say that the ad encourages violence against women because in their view prostitution is violence against women and vulnerable persons. By printing the ad in a newspaper or online you are encouraging violence against women. Freedom of expression would not be allowed because it gets trumped when violence is involved.
Damn, I should quit my job and work for Peter Mackay.
One argument against that is there are several fashion advertisements depicting violence against women while advertising a product.
Here's a link to Jezebel demonstrating it:
http://jezebel.com/5916650/fashions-ongoing-violence-against-women/
If these ads are allowed to be printed in magazines (for example) then escort ads should be allowed as well.