I ran into two JWs with a similar table awhile back. Instantly my heart rate shot up from the anticipation of approaching them. But I actually had about an hour and a half until my next appointment so I decided I would go up to them. The conversation started off simple, I eventually told them I had a friend that was an ex-JW. I obviously needed some kind of cover as to why I knew so much about the practices and history of their Cult.
My presence at their table certainly prevented curious people walking by from getting the full sales pitch. I was asking very difficult questions that raised the eyebrows of the few people that stopped while I was there.
After a slow progression of increasingly difficult questions, it became quite evident that these JWs would have preferred to be anywhere else but right there at that moment. But it's not so easy to pack up a whole table of books and run simply because someone is asking polite, yet very tough questions.
At one point the middle-aged JW women got up from the table and shook my hand saying, "It was nice to talk to you". Clearly she was trying to indicate to me to go away. Playing dumb, I responded, "It's been nice talking to you as well", and resumed asking questions of the other JW.
So while some may think these tables are annoying, others see them as a gift.