Why does Apple ship bash 3.2?
Yes, newer versions of Bash are released under GPLv3
This question brings out armchair lawyers that argue points for and against GPLv3. Simply put, GPLv3 conflicts with Apple's Licensing Policies (among other licenses). None of these licenses prevent you from downloading the source code and compiling a newer version of Bash. On a historical note, Bash was not originally offered as a shell in OS X.
Apple does not supply any software under the GPLv3 license.