maybe too safe?

Whether SRPM builds are allowed can be configured in Koji policy.
In Fedora koji, builds in -infra tags can be done from SRPM as far as I remember.
It's always possible to submit the build by using koji client directly in that case, but it seems like this change could be breaking an existing use case. Maybe allow some way to prevent this change?

Maybe I didn't explain the purpose of the feature request or I don't understand your point.
This PR tries to make commands

  rhpkg build --srpm
  rhpkg build --srpm --scratch

effectively the same. It simplifies arguments and for scratch-build, it is not required '--scratch' argument. It doesn't need any koji policy changes.
Which use cases could it break?

Check this task: https://koji.fedoraproject.org/koji/taskinfo?taskID=54939348
It's a non-scratch build from SRPM.

Thanks for this information. It breaks the original Bugzilla ticket idea that every SRPM build is a scratch-build. In the light of this fact, I am about to close this instead of implementing some exceptions for specific tags.