For iOS, at least, this is a requirement to conform with the license.
Since you cannot dynamically link with libraries on iOS, sparrow is using static linking. The letter of the lgpl requires that you provide the application in a way that it can be relinked - either using dynamic libraries (4.1) or providing the object files to relink (4.0) [1].
Taking a look at the OSX app, it looks like they link with only static libraries there, too, which puts it in the same boat.