While that may be common practice, it is unusual that there is so much data that not removing it is an issue. I agree it would be nice if the uninstaller mentioned that these files stay behind.
I agree and not only that, the Bitcoin Core program did not uninstall its data files either. But note there were two folders, each with about 28 to 27 GB of data:
c:\users\USERNAMEHERE\AppData\Roaming\Armory (28 GB data)
c:\users\USERNAMEHERE\AppData\Roaming\Bitcoin (27 GB data)
Both of these were not uninstalled and I had to manually delete them. If it was a less experienced user they would have left them there forever.