They did it before, much near the origins of the group. Remember the pictures of cars with the big speakers? People with signs hanging on their necks on the street? If memory serves me correctly (and please correct me if I am wrong) everything changed when Rutherford came around. I read somewhere that Rutherford paid his college education selling books door to door. He had experience in the business. He probably did not think public witnessing would do any good. Remember that JWs use to sell "subscriptions to paradise" in the not too distant past. With that model gone out the window and the bad numbers, they are resorting to old tactics I guess.
No, the cart does not have a money box. However, I remember clearly the days (not more than 5 years ago) when every demonstration had to be ended with the householder pretending to ask how much the mag was. To which we were supposed to reply that we take voluntary donations. I have not heard that in a long time, until about late 2014. Last time I've heard that again was just this weekend. In our last assembly, an example of witnessing on the street was presented and the above example was introduced as well.