I'm a webmaster since August 2000, and I learned HTML, phpBB, and other web related stuff all by myself and from the Net. But I never encountered web software that is forcing one to upgrade. Zenphoto is, because when not upgraded 3 or 4 weeks after the upgrade came out, my website displays the .../setup.php?autorun=gallery page.
Problem when trying to install 1.4.3.2: the message displayed several unnecessary files. After I clicked the button 'Delete Files', I'm getting a 500 error.
I checked the error log of my site, and some files are writeable by group so I changed their permissions to 644.

If you are not pleased with Zenphoto please do not use it. You certainly have not been FORCED to do anything.

No, Zenphoto does not force you to update and the message your encoutered does not say that. This behaviour is also documented (and was noted on release posts):
http://www.zenphoto.org/news/installation-and-upgrading#re-running-and-re-uploading-setup-files

If this is not a forced upgrade, then i don't know.

http://tv.muscleboykanan.com/1/

You have not been force to upgrade, unless you have uploaded a new release (your choice) no code has changed. What you are being forced to do is acknowledge that changes in your server environment have not impacted Zenphoto.

You are being asked to re-run the setup process on your existing installation. You would be forced to upgrade only if the change in your environment is incompatible with the release of Zenphoto you are running. Should that be the case, it is not Zenphoto that has forced the upgrade, it is your host or whom ever changed the environment.