Why do they charge?

The memorial is free, but there is an entry fee for the museum. The fees I guess is to maintain and upkeep the museum.

They charge to go inside the museum, but you don't have to pay to view the memorial fountains

Because it cost money to build the museum lol

You can enter the memorial site for free. Admission to the museum is free Tuesday evenings from 5pm to 8pm. The last admission is two hours before closing; however, limited quantities of these tickets are available. Four tickets are allowed per person. Family members of 9/11 victims and first responders and recovery workers will always have free admission to the museum.

I'm assuming for the upkeep of the museum and to pay their staff for operating it.

Veterans ate free

Admission to the museum is free Tuesday evenings from 5pm to 8pm.

You can walk to the memorial foe free but is you want to go in the museum you have to pay the tickets.

For the museum us $26 pp. Be prepare to walk

You Don't have to pay to walk around the outside but, when you go inside the museum you got to pay a fee. You can purchase the ticket from city pass.