The vendors are not on Port Authority Property, World Trade Center Property. They are allowed to sell the tasteless memorabila across the street from the Site.
As for the site itself, a couple months ago the Port Authority restored PATH service to the World Trade Center. The World Trade Center was built in the early 1970s above a major Commuter rail terminal that connects with New Jersey.
You can now go right down into the Trade Center site itself via the rail station, it's huge platforms offer unobstructed views from ground level. If you ride the PATH train it takes you right across the bottom of the site with unobstructed views, different perspective of looking up from the bottom of the "pit".
The PATH fare costs $1.50, I would highly recomend a ride to see it from the ground level. Take a ride to Hoboken
NJ which is two stops away, there are fantastic views of the Manhattan skyline from Frank Sinatra Park in Hoboken. Plus Hoboken is a young well to do town full of mostly 20/30 somethings who are recent college grads and work in Manhattan, the restaraunts along Washington street all have outdoor cafes and some of the best bars/clubs in the Tri-State area are in Hoboken.