It's not really the same as others being able to add tests to your automated suite. Rather, they add tests to their own package, and then the CI tool collects all tests indirectly dependent on your library into a virtual suite. Those tests are written to test their code, and only indirectly test yours. If a version of their package passes all of their tests with a previous version of your code, but the atomic change to the latest version of your code causes their test to fail, the failure was presumably caused by that change. The tests will probably have to be run multiple times to eliminate non-determinism.
It's still possible that someone writes code that depends on "features" that you consider to be bugs, or a pathologically sensitive test, so there may need to be some ability as the maintainer to flag tests as poor or unhelpful so they can be ignored in the future. Hopefully the requirement that the test pass the previous version to be considered is sufficient to cover most faulty tests though.
