apt packages delete /opt on uninstallation

[sblaisot]

Step to reproduce

1. start from a fresh debian or ubuntu install
2. apt-get install beamium
3. apt-get purge beamium
4. ls -d /opt

Expected result

/opt
/opt directory still exists

Actual results

ls: cannot access '/opt': No such file or directory

Removing beamium deletes /opt directory. Weird.