Removing tags in git

[Nick Schermer]

During the last exo release I added an invalid tag (well not entirely
invalid, but i did a pull --rebase before pushing, so the hash the tag
points to is not visible, when you click the tag in git the tarball
still contains the correct data). So I tried removing the tag with git
push origin :refs/tags/exo-0.5.1, but it seems this is not allowed by
our rules.
Talked about that with Jannis and he confirmed this was blocked on
purpose, but he couldn't remember why exactly.

So, why is removing tags not allowed? It seems to be normal to me to
correct mistakes like this or are there technical reasons?

Nick