The memorial is equipped with a museum shop, and as NPR notes, "Selling souvenirs at the memorial is prompting complex reactions, with some people complaining that a gift shop at such a hallowed site is in poor taste."
Perhaps it is wrong, and one could say that the shop is profiting off a national tragedy and the deaths of others.
On the other hand, the memorial is not funded by the government, and 60 to 70 percent of its budget comes from online ticket sales and the gift shop.
Accordingly, one could argue that people need a place to come and reflect on the awful events of that day, and a little commercialization is necessary in order for this to be possible.
At the same time, in many ways, it might be too soon to memorialize an event that we are still trying to come to terms with. The impact of 9/11, in terms of US foreign policy and the War on Terror, is still ongoing.
Basically, one could argue that a monument is meant to be a place for reflection, but are we really in a good position to begin wholeheartedly reflecting on 9/11?