While I agree with you that was not the case 15-20 years ago, I met some lovely girls on the streets and never encountered any trouble, now with the internet it is simply not worth the risk.
I think what has happened is that all the girls who might have been tempted by walking the street once in a while as a bill was coming in, all now have access to the internet. Lets face nearly everyone has, and even if you don;t have a connection at home there are plenty of other places to get access.
so instead of taking the risk and the danger associated with street walking they put up an advert on AW or go along to the local parlour and ask for a few shifts ( Parlours suddenly easy to find and contact due to listings on internet site etc)
The truth these days is it is only smackheads who are streetwalkers and I feel sorry for them as if you know anyone who has got involved with smack they all would give anything never to have touched the stuff in the first place. I feel genuinely sorry for them and I hope the find some help and a way out.
For a girl wanting to do a bit of prostitution so many more safer ways available than the street now.