As any other update operation performed directly on the view (as opposed to on the base data collection on which that view depends, see AsUpdatable<T>(View<T>)), it is allowed only if the view is not read-only (see IsReadOnly).

Although read-only properties of a view cannot be modified directly in the view, they still reflect up-to-date values of the source, so the difference is often not critical, you can always modify corresponding property in the source, that will automatically change the property in the view.