[taw28]

In short, because it was donated and not loaned. Once it leaves your hands, the museum can do as it wishes (within the restrictions of its bylaws).

[singleshot_]

“Philanthropic trust” is probably the concept you missed here. Don’t worry, it looks like Allen missed it too.

[ghaff]

And with fairly unusual exceptions, museums don't tend to like a lot of restrictions on what they can do with donations.