Being that it is near impossible to visit the 911 Memorial, you can visit the site through Google Earth, although you have to go through the rigmarole of installing the plug-in, which is almost as bad as trying to get a visitor's pass, or worse getting, through security once you get in.
Apparent the site is under such high security more for symbolic-emotional reasons since likelihood of the memorial being the target of a terrorist plot is close to nil:
To book [tickets], you’re obliged to provide your home address, email address, and phone number, and the full names of everyone in your party. It is “strongly recommended” that you print your tickets at home, which is where you must leave explosives, large bags, hand soap, glass bottles, rope, and bubbles. Also, “personal wheeled vehicles” not limited to bicycles, skateboards, and scooters, and anything else deemed inappropriate. Anyone age 13 or older must carry photo ID, to be displayed “when required and/or requested.” (Slate.com)