> First, why do people throw away tennis shoes, unopened food etc? Why not take with them on to their next destination?
It's very possible for the cost of transport to exceed the value of the goods, especially if one is flying to one's next destination.
> Second, why not just put on the street so that other people can come and collect items?
This I can't help with. As far as I have seen, this is a common practice everywhere in the northeast US. The police don't always love it, but that doesn't really prevent people from doing it.